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%I #12 Jul 14 2019 07:55:43
%S 1,1,5,15,56,203,749,2777,10293,38240,141997,527593,1960029,7282483,
%T 27057400,100531559,373522965,1387822193,5156442953,19158736256,
%U 71184183353,264484479633,982690786037,3651182836279,13565952140920,50404229548515,187276671274621
%N Number of perfect matchings on a triangular lattice of width 4 and length n.
%H S. N. Perepechko, <a href="http://dx.doi.org/10.13140/RG.2.2.20780.82568">Number of perfect matchings on triangular lattices of fixed width</a>, DIMA'2015 slides.
%F G.f.: (1-z)*(1+z)*(1-z-5*z^2-z^3+z^4)/((1+z-3*z^2-3*z^3+z^4)*(1-3*z-3*z^2+z^3+z^4)).
%Y Cf. A011900, A307349.
%K nonn,easy
%O 0,3
%A _Sergey Perepechko_, Jul 13 2019
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