Calculus/Points, paths, surfaces, and volumes

From Wikibooks, open books for an open world
Jump to navigation Jump to search

This chapter will provide an intuitive interpretation of vector calculus using simple concepts such as multi-points, multi-paths, multi-surfaces, and multi-volumes. Scalar fields will not be simply treated as a function that returns a number given an input point, and vector fields will not be simply treated as a function that returns a vector given an input point.


Contents

Basic structures[edit]

The basic structures are multi-points, multi-paths, multi-surfaces, and multi-volumes.

Multi-points[edit]

A point is an arbitrary position. A "multi-point" is a set of point/weight pairs: where is the "weight" that is assigned to point . Given two point/weight pairs and that cover the same point , the weights add up to get which replaces and . Any pair is removed. can consist of infinitely many points, and each point may have an infinitesimal weight.

An arbitrary point can be described by the scalar field . This is the "Dirac delta function" centered on point . The is the inverse of an infinitely small volume that wraps point . To further explain this, let be a tiny volume with volume that wraps point . can be approximated by . A mass of 1 is being crammed into yielding an infinitely high density. Since is essentially a density function, it brings with it the units .

Multi-point can be described by the scalar field . If consists of infinitely many points with each point having infinitesimal weight, then is a density function.

In the image below, the multi-point in the left panel is converted to the scalar field in the center panel by averaging the point weight over each cell. The volume of each cell should be infinitesimal. The multi-point in the right panel corresponds to the same scalar field, and is in a more canonical form where oppositely weighted points have cancelled out.

The multi-point (a collection of weighted points) on the left can be denoted by the scalar field in the middle. On the right is a more canonical multi-point with the same scalar field, where nearby points of opposite sign have cancelled out.

The image below shows how a continuous scalar field can be generated as a collection of points. Consider position and the infinitesimal volume with volume . The total point weight contained by is . This weight of is then split up over an arbitrarily large number of points that are scattered over the volume .

A single point of weight 1 can be "smeared out" over the volume that it sits in. The point is divided into an increasing number of points with fractional weights. After an infinite number of steps, there is an infinite number of points that fill the volume and each point has an infinitesimal weight.

In summary, a multi-point is denoted by a scalar field that quantifies the density at each point, and any scalar field that quantifies density at each point is best interpreted as a multi-point.

Multi-paths[edit]

A simple path (also called a simple curve) is an oriented continuous curve that extends from a starting point to an ending point . Intermediate points are indexed by and are denoted by . A simple path should be continuous (no breaks), and may intersect or retrace itself. A "multi-path" is a set of simple-path/weight pairs: where is the weight that is assigned to path . Given two path/weight pairs and that cover the same path , the weights add up to get which replaces and . Any pair is removed. In addition given two path/weight pairs and with the same weight and , then and can be linked end-to-end to get the pair which replaces and . Assigning a path a negative weight effectively reverses its orientation: if denotes path with the opposite orientation, then is equivalent to . can consist of infinitely many paths, and each path may have an infinitesimal weight.

This image depicts the Dirac delta function of a simple path. Unlike the Dirac delta function for a point which is a scalar field, the Dirac delta function for a path is a vector field.

An arbitrary curve can be described by the vector field . This is the "Dirac delta function" for the curve . is the unit length tangent vector to path at point . if . If there are multiple tangent vectors due to intersecting itself, then is the sum of these tangent vectors. The is the inverse of the cross-sectional area of an infinitely thin tube that encloses . To further explain this, let be a thin tube with cross-sectional area that encloses . can be approximated by . is the generalization of to the tube . A path weight of 1 is being crammed into the cross-sectional area of yielding an infinitely high path density. Since is essentially a density over area, it brings with it the units .

The image to the right gives a depiction of the Dirac delta function for a simple curve. The vector field is everywhere outside of an infinitely thin tube that encloses the path. Inside the tube, the vectors are parallel to the path, and have a magnitude equal to the inverse of the cross-sectional area. The Dirac delta function is the limit as the tube becomes infinitely thin.

Multi-path can be described by the vector field . If consists of infinitely many paths with each path having infinitesimal weight, then is a flow density function.

In the image below, the multi-path in the left panel is converted to the vector field in the center panel by computing the total displacement in each cell and averaging over the volume. The volume of each cell should be infinitesimal. The multi-path in the right panel corresponds to the same vector field, and is in a more canonical form where the individual paths do not cross each other.

The multi-path (a collection of weighted paths) on the left can be denoted by the vector field in the middle (in generating the vector field, each path was approximated to enter each cell through the middle of an edge). On the right is a more canonical multi-path with the same vector field, where nearby path segments with opposite orientations have cancelled out, and the individual paths do not cross each other.

In summary, a multi-path is denoted by a vector field that quantifies the path/flow density at each point, and any vector field that quantifies a flow density at each point (such as current density) is best interpreted as a multi-path. (Flow density is a vector that points in the net direction of a flow, and has a length equal to the flow rate per unit area through a surface that is perpendicular to the net flow.)

Multi-surfaces[edit]

A simple surface is an oriented continuous surface. A simple surface may intersect or fold back on itself. A "multi-surface" is a set of simple-surface/weight pairs: where is the weight that is assigned to surface . Given two surface/weight pairs and that cover the same surface , the weights add up to get which replaces and . Any pair is removed. In addition given two surface/weight pairs and with the same weight , then and can be combined to get the pair which replaces and . Assigning a surface a negative weight effectively reverses its orientation: if denotes surface with the opposite orientation, then is equivalent to . can consist of infinitely many surfaces, and each surface may have an infinitesimal weight.

An arbitrary surface can be described by the vector field . This is the "Dirac delta function" for the surface . is the unit length normal vector to surface at point . if . If there are multiple normal vectors due to intersecting itself, then is the sum of these normal vectors. The is the inverse of the thickness of an infinitely thin membrane that encloses . To further explain this, let be a thin membrane with thickness that encloses . can be approximated by . is the generalization of to the membrane . A surface weight of 1 is being sandwiched into the thickness of yielding an infinitely high surface density. Since is essentially a density over length, it brings with it the units .

Multi-surface can be described by the vector field . If consists of infinitely many surfaces with each surface having infinitesimal weight, then is a rate-of-gain function.

In the image below, the multi-surface in the left panel is converted to the vector field in the center panel by computing the total surface in each cell and averaging over the volume. The volume of each cell should be infinitesimal. The multi-surface in the right panel corresponds to the same vector field, and is in a more canonical form where the individual surfaces do not intersect each other.

The multi-surface (a collection of weighted surfaces) on the left can be denoted by the vector field in the middle (in generating the vector field, each surface was approximated to intersect the edge of each square in the middle). On the right is a more canonical multi-surface with the same vector field, where nearby surface segments with opposite orientations have cancelled out, and the individual surfaces do not intersect.

In summary, a multi-surface is denoted by a vector field that quantifies the rate of gain at each point. To describe the rate-of-gain, imagine that passing through a surface in the preferred direction gives "energy". The rate of gain is a vector that points in the direction that yields the greatest rate of energy increase per unit length, and has a length equal to the maximum rate of energy increase per unit length. Any vector field that quantifies a rate of gain at each point (such as a force field) is best interpreted as a multi-surface.

Multi-volumes[edit]

A volume is an arbitrary region of space. A "multi-volume" is a set of volume/weight pairs: where is the "weight" that is assigned to volume . Given two volume/weight pairs and that cover the same volume , the weights add up to get which replaces and . Any pair is removed. In addition given two volume/weight pairs and with the same weight and , then and can be combined to get the pair which replaces and . can consist of infinitely many volumes, and each volume may have an infinitesimal weight.

An arbitrary volume can be described by the scalar field . This is the "Dirac delta function" analog for volumes, and is essentially an indicator function that indicates whether or not a position is contained by or not, 1 being yes and 0 being no. Since is simply an indicator function, it brings with it no units (it is dimensionless).

Multi-volume can be described by the scalar field . If consists of infinitely many volumes with each volume having infinitesimal weight, then is a potential function.

In the image below, the multi-volume in the left panel is converted to the scalar field in the center panel by averaging the volume weight in each cell. The volume of each cell should be infinitesimal. The multi-volume in the right panel corresponds to the same scalar field, and is in a more canonical form where oppositely weighted volumes have cancelled out, and the remaining volume has diffused to fill each cell.

The multi-volume (a collection of weighted volumes) on the left can be denoted by the scalar field in the middle (in generating the scalar field, the beveled corners of each volume where ignored). On the right is a more canonical multi-volume with the same scalar field, where volumes of opposite sign have cancelled out, and the remaining volume is smeared out to fill each cell.

In summary, a multi-volume is denoted by a scalar field that quantifies a potential at each point, and any scalar field that quantifies a potential at each point is best interpreted as a multi-volume.

At infinity[edit]

An important requirement is that all multi-points, multi-paths, multi-surfaces, and multi-volumes not extend to infinity. All structures can extend to an arbitrarily large range, as long as this range is not unbounded. Allowing the structures to extend to infinity will cause problems in the later discussions.

Totals[edit]

These sections will describe the total weight of multi-points, the total displacement of multi-paths, the total surface of multi-surfaces, and the total volumes of multi-volumes.

Total point weight[edit]

Given a multi-point , the total point weight is clearly . Given a scalar field that denotes a multi-point, the total weight of is . Given a simple point , the total weight is 1 so .

Total displacement[edit]

The displacement between two points is independent of the path that connects them.

Given a simple path that starts at point and ends at point , the total displacement generated by is . This displacement is solely dependent on the endpoints as indicated by the top image to the right.

The displacement generated by a closed loop is .

Given a multi-path , the total displacement generated by is .

Given a vector field that denotes a multi-path, the total displacement generated by is . Since the displacement generated by a simple path is , it is the case that .

A path integral can be converted to a volume integral be replacing the displacement differential dq with the shown expression that is proportional to the volume differential dV. As is shown, the path is diffused to fill a thin tube. The integrand of the volume integral at all points outside of this thin tube is 0.

One important observation from is that given a path integral over path , the differential is equal to in a volume integral: provided that function is linear in the second parameter. In the lower image to the right, the displacement differential is equated to the volume differential by diffusing the path over an infintely thin cross-sectional area . The integrand at points outside of the infinitely thin tube is 0: for all points , .

Total surface vector[edit]

A flat surface with area "A", a counter-clockwise boundary denoted by the arrows, and an orientation out of the plane is depicted by this image. Normal vector "n" has a length of 1, is perpendicular to the surface, and is oriented out of the plane as shown. The surface itself can be described by the vector "A n". The length is the area, and the direction is the orientation.

Given an arbitrary oriented surface , its "counter-clockwise boundary", denoted by , is the boundary of whose orientation is determined in the following manner: Looking at so that the preferred direction through is oriented towards the viewer, the boundary wraps in a counter-clockwise direction.

Given a flat surface as shown in the image to the right, this surface can be quantified by the "surface vector" which is a vector that is perpendicular (normal) to the surface in the preferred orientation, and has a length equal to the area of the surface. In the image to the right, a flat surface has an area of and is oriented to be perpendicular to unit-length normal vector . The "surface vector" of this surface is .

Given a non-flat surface , the total surface vector of is computed by summing the surface vectors of each infinitesimal portion of . The total surface vector is .

In a manner similar to how the total displacement of a path is solely a function of the endpoints, the total surface vector of a surface is solely a function of its counter-clockwise boundary. This is not intuitive, and will be explained in greater detail below:

Two different surfaces are shown. Both surfaces have identical counter-clockwise boundaries, and because of this, the "total surface vector" for each surface are the same. Similar to how the total displacement along a path is purely a function of its endpoints, the total surface vector of a surface is purely a function of its boundary.

Below are shown two images related to surface vectors in 2D space. The image to the left shows surface vectors in 2D space. In 2 dimensions, surfaces are called 1D surfaces and are similar to paths. The boundary of a 1D surface consists of 2 points. The surface vector of a 1D surface segment is a 90 degree rotation of the segment and is oriented in the direction of the surface's orientation. The total surface vector of a 1D surface is the sum of all of the surface vectors of the individual components. For each component of the surface, the surface vector is a 90 degree rotation of the displacement that traverses the component, so the total surface vector is a 90 degree rotation of the displacement between the points that form the boundary of the surface. This proves that in two dimensions, the total surface vector depends only on the boundary of the 1D surface.

The image to the right extrudes the 1D surfaces in two dimensional space into 2D "ribbons" in 3 dimensional space. At the top a closed "ribbon" is shown. This "ribbon" is a surface that is always parallel to the vertical dimension, and whose boundary forms two identical loops that are vertically displaced from each other. The boundary loops are also perpendicular to the vertical dimension. The ribbon itself is partitioned into tiny rectangles whose height is equal to that of the ribbon. To the bottom left, a view of the same ribbon from the top down is shown. It can be seen that the length of the each surface vector is proportional to the length of the corresponding rectangular segment, since the heights are all uniform. To the bottom right, by rotating the surface vectors 90 degrees around the vertical dimension, the surface vectors now sum to , so the sum of the unrotated surface vectors is also .

This image depicts how in 2 dimensions, the total surface vector of a 1D surface is a 90 degree rotation of the displacement between the two endpoints (the boundary of a 1D surface), and is therefore purely a function of the endpoints. In the left panel, a 1D surface is a sequence of black line segments, and the surface vectors of each segment are denoted by the dashed red arrows. Each surface vector is a 90 degree rotation of the displacement along the surface. The long grey line is the net displacement between the endpoints of the surface, and the dashed pink arrow is a 90 degree rotation of this net displacement. In the right panel, the pink arrow is shown as the sum of the dashed red arrow vectors, hence the "total surface" is purely a function of the 1D surface's endpoints.
This image demonstrates that the total surface vector of a surface that is a closed ribbon is 0. The top image shows a surface that is a closed ribbon where the width of the ribbon is constant, the width is always parallel to the vertical dimension, and the edge is always perpendicular to the vertical dimension. The surface is sub-divided into tiny rectangular portions, the surface vectors of which are shown. The lower-left image shows the same surface from a top down perspective. In the lower-right image, the surface vectors are all rotated 90 degrees counter-clockwise around the vertical dimension and clearly sum to 0.

The fact that the total surface vector of a closed ribbon is means that if relief is added to a surface without changing its boundaries, the total surface vector is conserved. The two left images below give examples of distorting the interior of a surface by hammering in relief. The vertical surfaces introduced by the relief are ribbons which contribute to the total surface vector, while the horizontal surfaces are simply displaced vertically be the relief. The rightmost image below shows how the total surface vector is preserved if the "texture" of the surface at infinitesimal scales is converted from "steps" (a union of horizontal and vertical surfaces) to "smooth slopes" and vice versa. The surface formed from the red and green planes is a step, while the surface formed from the blue plane is a slope. These two surfaces can be seen to have equal total surface vectors from the right-angled triangle at the right side of the image.

Adding elevation (depression in this image) or relief to a surface does not change the total surface vector. The red colored horizontal surfaces are clearly conserved, albeit at different elevations. The green colored vertical surfaces sum to 0 at each tier/elevation.
Adding elevation (depression in this image) or relief to a surface does not change the total surface vector. The horizontal surfaces are clearly conserved, albeit at different elevations. The green colored vertical surfaces sum to their initial value above the lower red surface, and sum to 0 beneath the lower red surface.
In this image there are two surfaces. The first surface is the union of the red and green planes, and the counter-clockwise boundary is shown by the thick black line. The second surface is the blue plane and the counter-clockwise boundary is shown by the dashed blue line. The surface vectors of the red, green, and blue planes are shown. The total surface vector of the first surface is the sum of the surface vectors of the red and green planes, and is equal to the surface vector of the blue plane. This all implies that the total surface vector of a sloped flat surface is unchanged by replacing the surface with its horizontal and vertical components (projections).

The total surface vector generated by a closed surface is .

Given a multi-surface the total surface vector generated by is .

Given a vector field that denotes a multi-surface, the total surface vector generated by is . Since the surface vector generated by simple surface is , it is the case that . One important observation is that given a surface integral over , the differential is equal to in a volume integral: provided that function is a linear in the second parameter.

Total volume[edit]

Consider a multi-volume , where the volumes of are respectively , then the total volume of is . Each volume can be computed by . The total volume of is .

If a multi-volume can be denoted by scalar field , then the volume of is .

Given an arbitrary volume , a volume integral over can be converted to a volume integral over by replacing the differential with :

provided that is linear in the second parameter.

Intersections[edit]

The union of two multi-points denoted by scalar fields and is simply , and the same is true for the union of two multi-paths, the union of two multi-surfaces, and the union of two multi-volumes. The union of two structures with different types, such as a multi-point with a multi-path, is forbidden however.

Unions
structure multi-point multi-path multi-surface multi-volume
multi-point multi-point n/a n/a n/a
multi-path n/a multi-path n/a n/a
multi-surface n/a n/a multi-surface n/a
multi-volume n/a n/a n/a multi-volume

The intersection on the other hand, is less trivial and can occur between structures of different types.

Point-Volume intersections[edit]

When a point with weight intersects a volume with weight , then the intersection is point with weight . Given a multi-point and a multi-volume, the intersection is the sum of the pair-wise intersections of each simple point with each simple volume. The image below gives an example of the intersection of a multi-point with a multi-volume.

The left panel depicts both a multi-point and a multi-volume. The right panel depicts the intersection between the multi-point and the multi-volume, which is itself a multi-point. Note that points that intersect a volume with weight -1 have their weights flipped to their negative.

Given a multi-point with scalar field , and a multi-volume with scalar field , then the intersection is a multi-point with scalar field .

The total intersection between a multi-point and a multi-volume is .

If denotes a simple point , then the total intersection is .

If denotes a simple volume , then the total intersection is .

Path-Surface intersections[edit]

When a path with weight intersects a surface with weight at point , then the intersection is point with weight . The weight is if passes through in the direction in which is oriented. The weight is if passes through opposite the direction in which is oriented. Given a multi-path and a multi-surface, the intersection is the sum of the pair-wise intersections of each simple path with each simple surface. The images below give examples of the intersections of a multi-path with a multi-surface.

A 2D image showing the intersection of a multi-path (dark blue dashed curves) with a multi-surface (dark red solid curves). Positive intersection points (red circles) occur when a path intersects a surface in the preferred direction. Negative intersection points (teal circles) occur when a path intersects a surface in the opposite direction. The intersection is effectively a multi-point.
A 3D image showing the intersection of a simple path (red curve) with a simple surface (green surface with the counter-clockwise boundary highlighted). The positive intersection points are denoted by red "+" signs, and the negative intersection points are denoted by blue "-" signs.
The intersection between a multi-path shown as a blue tube with a multi-surface shown as layers of red sheets. Vector F is the flow density through the blue tube. Vector G is the surface density in the red sheets. The green parallelogram is a 2D projection of the volume of the intersection. The intersection points become more dilute as the angle theta increases, so the intersection point density is the dot product of F and G.

In the image above to the far right, the multi-path is denoted by a vector field which has the value inside the blue tube, and is everywhere else. The multi-surface is denoted by a vector field which has the value among the red sheets, and is everywhere else. The total path weight in the blue tube is . The total surface weight in the red sheets is . The total weight of all the intersection points is . The volume that the intersection points are evenly spread out in is . The intersection point density is .

Given a multi-path with vector field , and a multi-surface with vector field , then the intersection is a multi-point with scalar field .

The total intersection between a multi-path and a multi-surface is .

If is a simple path , then the total intersection is .

If is a simple surface , then the total intersection is .

Path-Volume intersections[edit]

When a path with weight intersects a volume with weight , then the intersection is path with weight . Given a multi-path and a multi-volume, the intersection is the sum of the pair-wise intersections of each simple path with each simple volume. The image below gives an example of the intersection of a multi-path with a multi-volume.

The left panel depicts both a multi-path and a multi-volume. The right panel depicts the intersection between the multi-path and the multi-volume, which is itself a multi-path. Note that the path's orientation is reversed in the negatively weighted volumes. In addition, the path segment in the weight 2 volume region in the middle has a weight of 2 as indicated by the thicker line.

Given a multi-path with vector field , and a multi-volume with scalar field , then the intersection is a multi-path with vector field .

The total intersection between a multi-path and a multi-volume is .

If denotes a simple path , then the total intersection is .

If denotes a simple volume , then the total intersection is .

Surface-Surface intersections[edit]

When a surface with weight intersects a surface with weight , then the intersection is the path with weight . The orientation given to path is defined as follows: viewing the intersection where the surface normal vectors of and are oriented towards the viewer, the intersection path has to its right, and to its left. Put another way, the intersection path is oriented according to the "right-hand rule" where the surface normals of are the "x" direction, and the surface normals of are the "y" direction. The images below give examples of the intersections of a multi-surface with a multi-surface.

A 3D image that shows the intersection of 2 surfaces. Surface 1 is blue and the normal vectors are oriented upwards. Surface 2 is red and the normal vectors are oriented to the right. The intersection is the black curve. The orientation of the intersection curve is determined via the right-hand rule with the surface normals of surface 1 as the "x" direction, and the surface normals of surface 2 as the "y" direction.
The intersection between two multi-surfaces. The first multi-surface is the layered blue sheets, and the second multi-surface is the layered red sheets. Vector F is the surface density in the blue sheets. Vector G is the surface density in the red sheets. The green parallelogram is a 2D cross-section of the prism that forms the intersection. The intersection paths become more dilute the further angle theta deviates from 90 degrees, so the intersection path density is the cross product of F and G. The intersection paths are also oriented out of the screen in this example.

In the image above to the right, the first multi-surface is denoted by a vector field that has the value among the blue sheets, and is everywhere else. The second multi-surface is denoted by a vector field that has the value among the red sheets, and is everywhere else. The total surface weight in the blue sheets is , and the total surface weight in the red sheets is . The total weight of all the intersection paths is . The cross-sectional area that the intersection paths are evenly spread out over is . The intersection path density is . Lastly, it should be noted that the intersection paths are oriented out of the screen as per the right-hand rule.

Given a multi-surface with vector field , and a multi-surface with vector field , then the intersection is the multi-path with vector field .

The total intersection between multi-surface and multi-surface is .

If denotes a simple surface , then the total intersection is .

Surface-Volume intersections[edit]

When a surface with weight intersects a volume with weight , then the intersection is surface with weight . Given a multi-surface and a multi-volume, the intersection is the sum of the pair-wise intersections of each simple surface with each simple volume. The image below gives an example of the intersection of a multi-surface with a multi-volume.

The left panel depicts both a multi-surface and a multi-volume. The right panel depicts the intersection between the multi-surface and the multi-volume, which is itself a multi-surface. Note that the surface's orientation is reversed in the negatively weighted volume. In addition, the surface segment in the weight 2 volume region in the upper-left has a weight of 2 as indicated by the thicker line.

Given a multi-surface with vector field , and a multi-volume with scalar field , then the intersection is a multi-surface with vector field .

The total intersection between a multi-surface and a multi-volume is .

If denotes a simple surface , then the total intersection is .

If denotes a simple volume , then the total intersection is .

Volume-Volume intersections[edit]

When a volume with weight intersects a volume with weight , then the intersection is the volume with weight . Given two multi-volumes, the intersection is the sum of the pair-wise intersections of each simple volume from the first multi-volume with each simple volume from the second multi-volume. The image below gives an example of the intersection between two multi-volumes.

The left two panels each depict a multi-volume, and the rightmost panel depicts the intersection of the two multi-volumes. The weight of the intersection of two simple volumes is the product of the weight of the two volumes.

Given a multi-volume with scalar field , and a multi-volume with scalar field , then the intersection is a multi-volume with scalar field .

The total intersection between multi-volume and multi-volume is .

If denotes a simple volume , then the total intersection is .

Other intersections[edit]

Other types of intersections, such as Point-Point intersections, Point-Path intersections, Point-Surface intersections, and Path-Path intersections, are not considered since these kinds of intersections can occur only by design. For example, the probability that two randomly chosen points will intersect each other is 0, but if a point and a volume are randomly chosen, then the probability of the point landing in the volume is nonzero. Given two unrelated points, these two points will never land on each other, since a prior relationship has to exist for the points to coincide. Below is summarized the various types of intersections:

Intersections
structure multi-point multi-path multi-surface multi-volume
multi-point n/a n/a n/a multi-point
multi-path n/a n/a multi-point multi-path
multi-surface n/a multi-point multi-path multi-surface
multi-volume multi-point multi-path multi-surface multi-volume

Boundaries[edit]

The endpoints of paths[edit]

Given a simple path that starts at point and ends at point , the "endpoints" of is the multi-point that consists of the starting point with a weight of +1, and the final point with a weight of -1. While is denoted by the vector field , the endpoints are denoted by the scalar field . The image below gives several examples of simple paths and their associated endpoints.

A series of panels, each depicting a directed path and its endpoints. The endpoints of a path consists of a positively weighted point at the start and a negatively weighted point at the end.

Given a multi-path , the endpoints of is the multi-point .

Given a vector field that denotes a multi-path, the multi-point that denotes the endpoints of is denoted by scalar field . The scalar field evaluated at point is denoted by , or .

The requirement that no path extends to infinity means that every starting point is paired with a final point, and therefore the total weight of all of the endpoints together is 0: .

The path endpoints are the intersections of the path with the "surface of reality".

The similarity of the notation to the intersection of multi-path with multi-surface , denoted by , makes sense if is interpreted as the "surface of reality". A starting point forms when a path pokes into reality, and a final point forms when a path pokes out of reality.

In the image to the right, a depiction of the "surface of reality" interpretation of is shown. On the right is a simple path , along with its endpoints . On the left is an extension of that is behind the "veil" of surface . pokes out of and into at points consistent with the endpoints of : i.e. .

The counter-clockwise oriented boundaries of surfaces[edit]

Given an oriented surface , the "counter-clockwise oriented boundary" of is a path that traces the boundary of in a counter-clockwise direction. The counter-clockwise direction is better described as follows: While located on the boundary, the counter-clockwise direction is the "forwards" direction when the surface normal vectors point "up" and the surface itself is on the "left". The image below gives several examples of oriented surfaces and their counter-clockwise boundaries. Note in particular the 4th panel that shows that the orientation around a hole in the surface appears to be clockwise.

A series of panels, each depicting an oriented surface and its counterclockwise oriented boundary. The surface normal vectors are depicted by the red arrows.

Given a multi-surface , the counter-clockwise boundary of is the multi-path .

Given a vector field that denotes a multi-surface, the multi-path that denotes the counter-clockwise boundary of is denoted by vector field . The vector field evaluated at point is denoted by , , or .

The requirement that no surface weight extends to infinity means that all counter-clockwise boundaries form closed loops, and therefore the total displacement of the total counter-clockwise boundary is : .

It is also important to note that the counter-clockwise boundary has no endpoints: .

The boundary of a surface is analogous to the intersection of the surface with the "surface of reality".

The similarity of the notation to the intersection of multi-surface with multi-surface , denoted by , again makes sense if is interpreted as the "surface of reality". An edge is formed when a surface "slices" into reality.

In the image to the right, a depiction of the "surface of reality" interpretation of is shown. On the right is a simple surface , along with its counter-clockwise boundary . On the left is an extension of that is behind the "veil" of surface . slices into at curves consistent with the boundary of : i.e. .

The inwards-oriented surfaces of volumes[edit]

Given a volume , the "inwards oriented surface" of is a surface that wraps the volume with the surface normals pointing inwards. The image below gives several examples of volumes and their inwards oriented surfaces.

A series of panels, each depicting a volume and its inwards oriented surface. The inwards orientation of the surface is indicated by the red arrows pointing inwards.

Given a multi-volume , the inwards oriented surface of is the multi-surface .

Given a scalar field that denotes a multi-volume, the multi-surface that denotes the inwards oriented surface of is denoted by vector field . The vector field evaluated at point is denoted by , , or .

The requirement that no volume weight extends to infinity means that all inwards oriented surfaces form closed surfaces, and therefore the total surface vector of the total inwards oriented surface is : .

It is also important to note that the inwards oriented surface has no boundary: .

In this 2D cross-section, the surface of a volume is analogous to the intersection of the volume with the "surface of reality".

The similarity of the notation to the intersection of multi-surface with multi-volume , denoted by , again makes sense if is interpreted as the "surface of reality". A surface is formed from the surface of reality when the volume "pushes" into reality.

In the image to the right, a depiction of the "surface of reality" interpretation of is shown. The image is a 2D cross-section for simplicity. On the right is a simple volume , along with its inwards oriented surface . On the left is an extension of that is behind the "veil" of surface . pushes through at surfaces consistent with the surface of : i.e. .


Coordinate Systems[edit]

This image depicts a generalized coordinate lattice at the top. At the bottom of the image is a single volume element with the basis displacement (contravariant) vectors, alongside the basis surface (covariant) vectors.

This section will describe how to compute various quantities such as intersections, endpoints, boundaries, and surfaces given a curvilinear coordinate system.

Let the curvilinear coordinate system be arbitrary. Let the 3 coordinates that index all points be . Coordinates will be denoted by the triple .

The following notation will be used in the following discussions:

  • Given an arbitrary expression that assigns a real number to each index , then will denote the triple .
  • Given index variables , the expression equals 1 if and 0 if otherwise.
  • Given an arbitrary expression that assigns a real number to each index , then will denote the sum .
  • Given an index variable , will rotate forwards by 1, and will rotate forwards by 2. In essence, and .

Start with an arbitrary coordinate and infinitesimal differences , , and . The following 3 paths, 3 surfaces, and volume will be associated with point :

  • For each there exists an infinitely short path starting from point and ending on point along the curve defined by , and . The displacement covered by is approximately where is a unit length vector that is parallel to the displacement between points and , and is the length of the displacement. Note that the length of the displacement is proportional to , with being the constant of proportionality.
  • For each there exists an infinitely small surface that is defined by the following: , , and . The orientation of is in the direction of increasing . The surface vector of is approximately where is a unit length vector that is perpendicular to , and is the area of . Note that the area of is proportional to , with being the constant of proportionality.
  • There is an infinitely small volume defined by , , and . has a shape that is approximately that of a parallelepiped. The volume of is approximately . Note that the volume of is proportional to , with being the constant of proportionality.

It is important to note that:

  • if and only if , , and (note the strictness of the upper bounds).
  • For all , if and only if , , and (note the strictness of the upper bounds).
  • For all , if and only if (note the strictness of the upper bound), , and .

Converting between multi-points, multi-paths, multi-surfaces, and multi-volumes and their respective scalar fields and vector fields proceeds as follows:

Converting to and from multi-points and scalar fields[edit]

Given an arbitrary multi-point , the scalar field that denotes can be approximated as follows: Choose arbitrary infinitesimal differences , , . Given an arbitrary point , let . Let . contains point . where is the total point weight contained by .

Given an arbitrary scalar field , a multi-point that is denoted by can be approximated as follows: Choose arbitrary infinitesimal differences , , . For any triple of integers , there is a point created at the weight of this point is .

Converting to and from multi-paths and vector fields[edit]

Given an arbitrary multi-path , the vector field that denotes can be approximated as follows: Choose arbitrary infinitesimal differences , , . Given an arbitrary point , let . Let . contains point . where is the total path displacement contained by .

Given an arbitrary vector field , a multi-path that is denoted by can be approximated as follows: Choose arbitrary infinitesimal differences , , . Express as a linear combination of the displacement (contravariant) basis vectors , , and : . For any triple of integers and , consider path . The weight assigned to is .

Converting to and from multi-surfaces and vector fields[edit]

Given an arbitrary multi-surface , the vector field that denotes can be approximated as follows: Choose arbitrary infinitesimal differences , , . Given an arbitrary point , let . Let . contains point . where is the total surface vector contained by .

Given an arbitrary vector field , a multi-surface that is denoted by can be approximated as follows: Choose arbitrary infinitesimal differences , , . Express as a linear combination of the surface (covariant) basis vectors , , and : . For any triple of integers and , consider surface . The weight assigned to is .

Converting to and from multi-volumes and scalar fields[edit]

Given an arbitrary multi-volume , the scalar field that denotes can be approximated as follows: Choose arbitrary infinitesimal differences , , . Given an arbitrary point , let . Let . contains point . where is the total volume weight contained by .

Given an arbitrary scalar field , a multi-volume that is denoted by can be approximated as follows: Choose arbitrary infinitesimal differences , , . For any triple of integers , the volume has the weight .

Computing various intersections[edit]

Computing the intersection of any structure with a multi-volume is trivial matter: Simply multiply the scalar of vector field by the scalar field that denotes the multi-volume. When both structures are denoted by vector fields however, computing the intersection is far less trivial.

Computing path-surface intersections[edit]

Given a multi-path denoted by vector field , and a multi-surface denoted by vector field the scalar field that denotes the intersection can be computed as follows:

For any triple of integers and , let . The weight assigned to by is approximately , and the weight assigned to by is approximately .

The intersection between and is point with weight .

Aside from the intersections between and for each triple of integers and , no other intersections occur. The total weight of the intersection at point is .

Given an arbitrary point , let , and . The value of at is approximately