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A203320 G.f.: exp( Sum_{n>=1} x^n/n * exp( Sum_{k>=1} sigma(n*k) * x^(n*k)/k ) ). 4
1, 1, 2, 4, 9, 16, 35, 61, 124, 222, 427, 749, 1434, 2493, 4585, 8032, 14511, 25096, 44791, 77019, 135435, 232002, 402957, 685582, 1181399, 1998168, 3410288, 5741978, 9726821, 16286497, 27409625, 45672026, 76378731, 126706567, 210690588, 347954716, 575685559, 946756712 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,3

LINKS

Paul D. Hanna, Table of n, a(n) for n = 0..120

FORMULA

G.f.: exp( Sum_{n>=1} P_n(x^n) * x^n/n ) where P_n(x^n) = Product_{k=0..n-1} P(u^k*x), u is an n-th root of unity, and P(x) is the partition function (A000041); P(x) = exp(Sum_{n>=1} sigma(n)*x^n/n) where sigma(n) is the sum of divisors of n (A000203).

The logarithmic derivative yields A203321.

EXAMPLE

G.f.: A(x) = 1 + x + 2*x^2 + 4*x^3 + 9*x^4 + 16*x^5 + 35*x^6 + 61*x^7 +...

G.f.: A(x) = exp( Sum_{n>=1} P_n(x^n) * x^n/n )

where P_n(x) = exp( Sum_{k>=1} sigma(n*k)*x^k/k ), which begin:

P_1(x) = 1 + x + 2*x^2 + 3*x^3 + 5*x^4 + 7*x^5 + 11*x^6 + 15*x^7 +...;

P_2(x) = 1 + 3*x + 8*x^2 + 19*x^3 + 41*x^4 + 83*x^5 + 161*x^6 +...;

P_3(x) = 1 + 4*x + 14*x^2 + 39*x^3 + 101*x^4 + 238*x^5 + 533*x^6 +...;

P_4(x) = 1 + 7*x + 32*x^2 + 119*x^3 + 385*x^4 + 1127*x^5 + 3057*x^6 +...;

P_5(x) = 1 + 6*x + 27*x^2 + 98*x^3 + 315*x^4 + 917*x^5 + 2486*x^6 +...;

P_6(x) = 1 + 12*x + 86*x^2 + 469*x^3 + 2141*x^4 + 8594*x^5 +...;

P_7(x) = 1 + 8*x + 44*x^2 + 192*x^3 + 726*x^4 + 2464*x^5 +...;

P_8(x) = 1 + 15*x + 128*x^2 + 815*x^3 + 4289*x^4 + 19663*x^5 +...;

...

Also, P_n(x^n) = Product_{k=0..n-1} P(u^k*x) where u = n-th root of unity:

P_1(x) = P(x), the partition function;

P_2(x^2) = P(x)*P(-x);

P_3(x^3) = P(x)*P(u*x)*P(u^2*x) where u = exp(2*Pi*I/3);

P_4(x^4) = P(x)*P(I*x)*P(I^2*x)*P(I^3*x) where I^2 = -1;

...

The logarithmic derivative of this sequence begins:

A203321 = [1,3,7,19,26,75,78,211,241,518,463,1447,1002,2558,...].

PROG

(PARI) {a(n)=local(L=vector(n+1, i, 1)); L=Vec(deriv(sum(m=1, n, x^m/m*exp(sum(k=1, floor(n/m), sigma(m*k)*x^(m*k)/k)+x*O(x^n))))); polcoeff(exp(x*Ser(vector(n+1, m, L[m]/m))), n)}

(PARI) {a(n)=local(A=1+x+x*O(x^n), P=exp(sum(k=1, n, sigma(k)*x^k/k)+x*O(x^n))); A=exp(sum(m=1, n+1, x^m/m*round(prod(k=0, m-1, subst(P, x, exp(2*Pi*I*k/m)*x+x*O(x^n)))))); polcoeff(A, n)}

CROSSREFS

Cf. A203321; A000041, A182818, A182819, A182820, A182821.

Sequence in context: A091437 A131337 A062791 * A005980 A128093 A056331

Adjacent sequences:  A203317 A203318 A203319 * A203321 A203322 A203323

KEYWORD

nonn

AUTHOR

Paul D. Hanna, Dec 31 2011

STATUS

approved

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Last modified July 31 10:51 EDT 2021. Contains 346373 sequences. (Running on oeis4.)