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a(n)=-1/b(2n) where 1/(e^y-e^(y/3))= sum(i=-1,inf,b(i)*y^i).
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%I #5 Mar 30 2012 18:38:52

%S 1,18,1944,524880,264539520,214277011200,254561089305600,

%T 416971064282572800,900657498850357248000,2480410751833883860992000,

%U 8483004771271882804592640000,35272333838948488701496197120000

%N a(n)=-1/b(2n) where 1/(e^y-e^(y/3))= sum(i=-1,inf,b(i)*y^i).

%F a(n)=A007661(6n+1)=A032031(3n+1)

%K easy,nonn

%O 0,2

%A _Benoit Cloitre_, Mar 12 2002