%I #8 Feb 06 2017 18:23:23
%S 1,1,1,1,1,1,1,53,141,271,449,681,973,1331,9391,26803,60953,122641,
%T 226921,394031,650413,2039288,5611209,13694084,30417897,62763334,
%U 122046063,225958222,532867801,1319078776,3196149101,7383041514,16186545853,33788903208
%N Number of tilings of a 21 X n rectangle using 3n heptominoes of shape I.
%H Alois P. Heinz, <a href="/A251076/b251076.txt">Table of n, a(n) for n = 0..500</a>
%H Wikipedia, <a href="https://en.wikipedia.org/wiki/Heptomino">Heptomino</a>
%Y Column k=7 of A251072.
%Y Cf. A250664.
%K nonn
%O 0,8
%A _Alois P. Heinz_, Nov 29 2014
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