This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A217745 #7 Mar 23 2013 15:10:04 %S A217745 1,4,10,18,35,64,117,190,330,492,780,1050,1605,2080,2941 %N A217745 Number of regions in the exterior of a regular n-gon with all diagonals drawn. %e A217745 a(3) = 1 since the equilateral triangle has no diagonals and therefore one exterior region (with infinite area). %e A217745 a(4) = 4 since the two diagonals of the square divide the exterior in four regions (with infinite area). %e A217745 a(5) = 10 since the ten diagonals of the regular pentagon divide the exterior in ten regions (with infinite area) of two different shapes. %Y A217745 Cf. A007678. %K A217745 nonn %O A217745 3,2 %A A217745 _Martin Renner_, Mar 23 2013