OFFSET
2,1
LINKS
S. N. Perepechko, The number of perfect matchings in one kind of strip graphs (in Russian), Information Processes, 14 (2014), 340-356.
FORMULA
a(n) = 2*product_{j=1..n} (80 - 98*cos((2*j-1)*Pi/n) + 24*cos(2*(2*j-1)*Pi/n) - 2*cos(3*(2*j-1)*Pi/n)).
G.f.: 8*x^2*(784 - 67669*x + 2453871*x^2 - 50439798*x^3 + 665164698*x^4 - 6023289070*x^5 + 39096248258*x^6 - 187328171158*x^7 + 676655443050*x^8 - 1870967276271*x^9 + 4004062704149*x^10 - 6684136860372*x^11 + 8747997318284*x^12 - 9001233440740*x^13 + 7286680504380*x^14 - 4634602342804*x^15 + 2308061094588*x^16 - 894754403811*x^17 + 267700931657*x^18 - 61077759670*x^19 + 10454781914*x^20 - 1313064750*x^21 + 117311490*x^22 - 7125462*x^23 + 273866*x^24 - 5849*x^25 + 51*x^26)/((1-x)*(1-4*x+x^2)*(1-14*x+34*x^2-14*x^3+x^4)* (1-8*x+16*x^2-8*x^3+x^4) * (1-56*x+672*x^2-2632*x^3+4094*x^4-2632*x^5+672*x^6-56*x^7+x^8)* (1-32*x+288*x^2-928*x^3+1346*x^4-928*x^5+288*x^6-32*x^7+x^8)).
CROSSREFS
KEYWORD
nonn
AUTHOR
Sergey Perepechko, Feb 03 2015
STATUS
approved