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 A255261 #5 Feb 20 2015 10:53:47 %S A255261 1,2,3,4,5,6,13,20,27,34,41,48,55,62,69,76,83,90,97,104,111,118,125, %T A255261 132,139,146,153,313,473,633,793,953,1113,1273,1433,1593,1753,1913, %U A255261 2073,2233,2393,2553,2713,2873,3033,3193,3353,3513,3673,3833,3993,4153 %N A255261 a(n) = a(n-1) + sum of the hexagonal numbers which are among the first (n-1) terms of the sequence, with a(1)=1. %H A255261 Colin Barker, <a href="/A255261/b255261.txt">Table of n, a(n) for n = 1..1000</a> %e A255261 a(8) = 20 because a(7) = 13, hexagonal numbers already in the sequence are 1 and 6, and 13+1+6 = 20. %o A255261 (PARI) vector(100, n, if(n==1, a_n_1=sum_=1, a_n_1+=sum_; if(ispolygonal(a_n_1, 6), sum_+=a_n_1)); a_n_1) %Y A255261 Cf. A100963, A101135, A255260. %K A255261 nonn %O A255261 1,2 %A A255261 _Colin Barker_, Feb 20 2015