# Puzzles/Easy Sequence 2/Solution

Add 1 2 3 4... OR x(x-1)/2+1 :

1 2 4 7 11 16 22 29 ...

${\displaystyle a_{n}=a_{n-2}+a_{n-1}+1}$:

1 2 4 7 12 20 33 ...

A formula for the ${\displaystyle n}$th number is: ${\displaystyle a_{0}=1}$, ${\displaystyle a_{2n}=a_{n}+2n}$, ${\displaystyle a_{2n+1}=6n+3-a_{n}}$.