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%I #32 Aug 13 2015 22:01:35
%S 1,7,56,252,1260,4620,18480,59815,206780,605934,1869840,5038572,
%T 14164920,35459424,91871208,214864650,516936420,1134183050,2546423880,
%U 5252058812,11031780760,21406686756,42121344720,76871042612,141627466344,242619996500,417750499600
%N Number of possible positions for n discs on a standard 7 X 6 board of Connect-Four.
%H 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 Victor Allis, <a href="/A090224/a090224.pdf">A Knowledge-Based Approach of Connect-Four--The Game Is Solved: White Wins</a>.
%H John Tromp, <a href="http://tromp.github.io/c4/c4.html">John's Connect Four Playground</a>, viewed Jan 23 2014.
%H Eric Weisstein's World of Mathematics, <a href="http://mathworld.wolfram.com/Connect-Four.html">Connect-Four</a>
%H Wikipedia, <a href="https://en.wikipedia.org/wiki/Connect_Four">Connect Four</a>
%e 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 Cf. A212693 (legal positions after n plies).
%K nonn,fini,full,nice
%O 0,2
%A _Eric W. Weisstein_, Nov 24 2003
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