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 A250400 #7 Nov 21 2014 07:48:02 %S A250400 1,16,504,1680,27300,87360,1936480,24344320,1232431200,1369368000, %T A250400 46695448800,865988323200,34711698621600,37381829284800, %U A250400 1722234277764000,21126073807238400,157125173941335600,55455943744000800,5321529955232400,162446703896568000 %N A250400 Numerator of the harmonic mean of the first n octagonal numbers. %H A250400 Colin Barker, <a href="/A250400/b250400.txt">Table of n, a(n) for n = 1..1000</a> %e A250400 a(3) = 504 because the first 3 octagonal numbers are [1,8,21], and 3/(1/1+1/8+1/21) = 504/197. %o A250400 (PARI) %o A250400 harmonicmean(v) = #v / sum(k=1, #v, 1/v[k]) %o A250400 s=vector(30); for(n=1, #s, s[n]=numerator(harmonicmean(vector(n, k, 3*k^2-2*k)))); s %Y A250400 Cf. A000567 (octagonal numbers), A250401 (denominators). %K A250400 nonn %O A250400 1,2 %A A250400 _Colin Barker_, Nov 21 2014