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 A090224 #33 Feb 16 2025 08:32:51 %S A090224 1,7,56,252,1260,4620,18480,59815,206780,605934,1869840,5038572, %T A090224 14164920,35459424,91871208,214864650,516936420,1134183050,2546423880, %U A090224 5252058812,11031780760,21406686756,42121344720,76871042612,141627466344,242619996500,417750499600 %N A090224 Number of possible positions for n discs on a standard 7 X 6 board of Connect-Four. %H A090224 Alois P. Heinz, <a href="/A090224/b090224.txt">Table of n, a(n) for n = 0..42</a> (copied from the Victor Allis link) %H A090224 Victor Allis, <a href="/A090224/a090224.pdf">A Knowledge-Based Approach of Connect-Four--The Game Is Solved: White Wins</a>. %H A090224 John Tromp, <a href="http://tromp.github.io/c4/c4.html">John's Connect Four Playground</a>, viewed Jan 23 2014. %H A090224 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/Connect-Four.html">Connect-Four</a> %H A090224 Wikipedia, <a href="https://en.wikipedia.org/wiki/Connect_Four">Connect Four</a> %e A090224 a(2) = 2 * binomial(8,2) since there are binomial(8,2) ways to distribute 2 discs over 7 columns and 2 ways to color them black and white. - _John Tromp_, May 24 2012 %Y A090224 Cf. A212693 (legal positions after n plies). %K A090224 nonn,fini,full,nice %O A090224 0,2 %A A090224 _Eric W. Weisstein_, Nov 24 2003