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a(n) = (2^(n-1)/(2n)!)*Product_{k=1..n} q(k) where q(n) is the denominator of B(2n), the 2n-th Bernoulli number.
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%I #11 Jan 04 2021 07:47:54

%S 3,15,42,45,66,2730,180,765,3990,6930,4140,40950,756,1740,57288,58905,

%T 630,1919190,16380,284130,595980,434700,118440,4873050,262548,314820,

%U 175560,99180,21240,681440760,2162160,546975,16504110,217350,421740

%N a(n) = (2^(n-1)/(2n)!)*Product_{k=1..n} q(k) where q(n) is the denominator of B(2n), the 2n-th Bernoulli number.

%C Lim_{n->inf} a(n)^(1/n) = 1.

%o (PARI) a(n) = (2^(n-1)/(2*n)!)*prod(k=1, n, denominator(bernfrac(2*k))); \\ _Michel Marcus_, Jan 04 2021

%Y Cf. A002445.

%K easy,nonn

%O 1,1

%A _Benoit Cloitre_, Apr 14 2002

%E Name edited by _Michel Marcus_, Jan 04 2021