# On 2D Inverse Problems/Y-Δ and star-mesh transforms

Exercise (**). Let d be a diagonal entry of the Kirchhoff matrix K of a network G, corresponding to an interior node. Use the Schur complement formula ${\displaystyle \Lambda _{G}=K/C=(K/d)/(C/d)}$ for the Dirichlet-to-Neumann map to prove the invariance.