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 A206764 a(n) = Sum_{k=1..n} binomial(n,k) * sigma(n,k) * (-1)^(n-k). 1

%I

%S 1,-1,10,79,1026,15686,279938,5771359,134218243,3487832974,

%T 100000000002,3138673052878,106993205379074,3937454749863382,

%U 155568096631586820,6568441588686506943,295147905179352825858,14063102470280932000757,708235345355337676357634

%N a(n) = Sum_{k=1..n} binomial(n,k) * sigma(n,k) * (-1)^(n-k).

%C Here sigma(n,k) equals the sum of the k-th powers of the divisors of n.

%e L.g.f.: L(x) = x - x^2/2 + 10*x^3/3 + 79*x^4/4 + 1026*x^5/5 + 15686*x^6/6 +...

%e Exponentiation yields the g.f. of A206763:

%e exp(L(x) = 1 + x + 3*x^3 + 23*x^4 + 225*x^5 + 2824*x^6 + 42670*x^7 +...

%e Illustration of terms.

%e a(2) = -2*sigma(2,1) + 1*sigma(2,2) = -2*3 + 1*5 = -1;

%e a(3) = 3*sigma(3,1) - 3*sigma(3,2) + 1*sigma(3,3) = 3*4 - 3*10 + 1*28 = 10;

%e a(4) = -4*sigma(4,1) + 6*sigma(4,2) - 4*sigma(4,3) + 1*sigma(4,4) = -4*7 + 6*21 - 4*73 + 1*273 = 79.

%o (PARI) {a(n)=sum(k=1, n, binomial(n, k)*sigma(n, k)*(-1)^(n-k))}

%o (PARI) {a(n)=n*polcoeff(sum(k=1, n, (1/k)*log((1-(-x)^k)/(1-(k-1)^k*x^k +x*O(x^n)))), n)}

%o for(n=1,21,print1(a(n),", "))

%Y Cf. A206763 (exp), A205815, A205812.

%K sign

%O 1,3

%A _Paul D. Hanna_, Feb 12 2012

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Last modified December 11 21:15 EST 2019. Contains 329937 sequences. (Running on oeis4.)