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 A120744 #11 Feb 16 2025 08:33:02 %S A120744 2,-1,1,3,2,7,15,1,16,8,14,4,5,15,1,2,5,-1,6,3,2,39,6,1,21,7,110,3,15, %T A120744 7,15,-1,2,8,1,4,989,8,14,2,45,15,9,4,5,335,9,1,29,-1,30,15,10,415,6, %U A120744 2,10,32,54,3,77,55,1,5,2,7,47750,11,15,23,47,-1,48,24,16,12,5,8,2639,1,6720,704,38,4,2,39,505,3,13,56,9,20,13,1631,41 %N A120744 Least k>0 such that a centered polygonal number nk(k+1)/2+1 is a perfect square; or -1 if no such number exists. %H A120744 Max Alekseyev, <a href="/A120744/b120744.txt">Table of n, a(n) for n = 1..10000</a> %H A120744 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/CenteredPolygonalNumber.html">Centered Polygonal Number</a> %F A120744 a(n) = -1 for n in A166259. %F A120744 a(n) = 1 for n = k^2-1. %e A120744 a(5) = 2 because A129556(2) = 2>1 and A129556(1) = 0<1. %Y A120744 Cf. A001542, A006451, A001921, A129444, A001570, A001652, A129556, A053606, A105038, A105040, A053141, A061278. %K A120744 sign %O A120744 1,1 %A A120744 _Alexander Adamchuk_, Apr 26 2007 %E A120744 Edited and b-file provided by _Max Alekseyev_, Jan 20 2010