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(-1)^(n+1)/3*sum(k=1,2n,C(2n+1,k)*B(k)*6^k) where C(n,k) are the binomial coefficients, B(k) the Bernoulli numbers.
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%I #5 Mar 30 2012 18:38:58

%S 3,57,2123,138609,13883283,1974243241,378027128283,93761532169953,

%T 29240923237509923,11199084302796542745,5167442341466021937003,

%U 2827285169201754895553617,1809878752362221218378054323

%N (-1)^(n+1)/3*sum(k=1,2n,C(2n+1,k)*B(k)*6^k) where C(n,k) are the binomial coefficients, B(k) the Bernoulli numbers.

%o (PARI) for(n=1,25,print1(-(-1)^n*sum(i=1,2*n,binomial(2*n+1,i)*bernfrac(i)*6^i)/3,","))

%K easy,nonn

%O 1,1

%A _Benoit Cloitre_, May 01 2002