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A309117
Number of perfect matchings on a triangular lattice of width 4 and length n.
0
1, 1, 5, 15, 56, 203, 749, 2777, 10293, 38240, 141997, 527593, 1960029, 7282483, 27057400, 100531559, 373522965, 1387822193, 5156442953, 19158736256, 71184183353, 264484479633, 982690786037, 3651182836279, 13565952140920, 50404229548515, 187276671274621
OFFSET
0,3
FORMULA
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)).
CROSSREFS
Sequence in context: A137959 A001552 A165731 * A307349 A343841 A203294
KEYWORD
nonn,easy
AUTHOR
Sergey Perepechko, Jul 13 2019
STATUS
approved