Table of n, a(n) for n=3..17.
a(3) = 1 since the equilateral triangle has no diagonals and therefore one exterior region (with infinite area).
a(4) = 4 since the two diagonals of the square divide the exterior in four regions (with infinite area).
a(5) = 10 since the ten diagonals of the regular pentagon divide the exterior in ten regions (with infinite area) of two different shapes.
Sequence in context: A050188 A062361 A038416 * A213949 A009913 A063591
Adjacent sequences: A217742 A217743 A217744 * A217746 A217747 A217748
Martin Renner, Mar 23 2013