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a(n)=(-1)^(n+1)*(7/8)*(49^n-1)*B(2n) where B(k) denotes the k-th Bernoulli number.
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%I #7 May 24 2016 11:16:19

%S 0,7,70,2451,168140,18724685,3065490210,692352720199,206231512032280,

%T 78326336951247393,36942519649032690350,21183831880266815272667,

%U 14513757566643671668200420,11709262576429836293006370421

%N a(n)=(-1)^(n+1)*(7/8)*(49^n-1)*B(2n) where B(k) denotes the k-th Bernoulli number.

%t Table[(-1)^(n+1)*7/8*(49^n-1)*BernoulliB[2n],{n,0,20}] (* _Harvey P. Dale_, May 24 2016 *)

%o (PARI) a(n)=(-1)^(n+1)*(7/8)*(49^n-1)*bernfrac(2*n)

%K nonn

%O 0,2

%A _Benoit Cloitre_, Dec 13 2003