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 A255260 #9 Feb 20 2015 10:53:37 %S A255260 1,2,3,4,5,11,17,23,29,35,76,117,275,433,591,749,907,1065,1223,1381, %T A255260 1539,1697,1855,2013,2171,2329,2487,2645,2803,2961,3119,3277,3435, %U A255260 3593,3751,3909,4067,4225,4383,4541,4699,4857,5015,5173,5331,5489,5647,5805 %N A255260 a(n) = a(n-1) + sum of the pentagonal numbers which are among the first (n-1) terms of the sequence, with a(1)=1. %H A255260 Colin Barker, <a href="/A255260/b255260.txt">Table of n, a(n) for n = 1..1000</a> %e A255260 a(7) = 17 because a(6) = 11, pentagonal numbers already in the sequence are 1 and 5, and 11+1+5 = 17. %o A255260 (PARI) vector(100, n, if(n==1, a_n_1=sum_=1, a_n_1+=sum_; if(ispolygonal(a_n_1, 5), sum_+=a_n_1)); a_n_1) %Y A255260 Cf. A100963, A101135, A255261. %K A255260 nonn %O A255260 1,2 %A A255260 _Colin Barker_, Feb 19 2015