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%I #10 Dec 07 2018 18:04:44
%S 3,21,120,525,2268,12936,41184,199485,948090,3879876,12697776,
%T 81124680,218412600,1123264800,5584230720,18934032285,63007367940,
%U 412918656150,1060357914000,6203093796900,25836377973120,88372156476240,296403506193600,1999351428352200,5878093199355468,24300008114457096,116816365538886720,458921436045626400,1353026992479346800
%N a(n) = sigma(2*n) * binomial(2*n,n)/2, for n >= 1.
%C Related logarithmic series:
%C (1) log( Product_{n>=1} (1 - x^(2*n))/(1 - x^n)^3 ) = Sum_{n>=1} sigma(2*n) * x^n/n (see formula of _Joerg Arndt_ in A182818).
%C (2) log( C(x) ) = Sum_{n>=1} binomial(2*n,n)/2 * x^n/n, where C(x) = 1 + x*C(x)^2 is the Catalan function (A000108).
%H Paul D. Hanna, <a href="/A322185/b322185.txt">Table of n, a(n) for n = 1..512</a>
%F a(n) is the coefficient of x^n*y^n/n in log( Product_{n>=1} 1/(1 - (x + y)^n) ), for n >= 1.
%e L.g.f: L(x) = 3*x + 21*x^2/2 + 120*x^3/3 + 525*x^4/4 + 2268*x^5/5 + 12936*x^6/6 + 41184*x^7/7 + 199485*x^8/8 + 948090*x^9/9 + 3879876*x^10/10 + 12697776*x^11/11 + ... + sigma(2*n) * binomial(2*n,n)/2 * x^n/n + ...
%e RELATED SERIES.
%e exp(L(x)) = 1 + 3*x + 15*x^2 + 76*x^3 + 357*x^4 + 1662*x^5 + 8203*x^6 + 36609*x^7 + 169800*x^8 + 788024*x^9 + 3586350*x^10 + 15948147*x^11 + ... + A322186(n)*x^n + ...
%e The table of coefficients of x^n*y^k/(n+k) in
%e log( Product_{n>=1} 1/(1 - (x + y)^n) ) = (1*x + 1*y)/1 + (3*x^2 + 6*x*y + 3*y^2)/2 + (4*x^3 + 12*x^2*y + 12*x*y^2 + 4*y^3)/3 + (7*x^4 + 28*x^3*y + 42*x^2*y^2 + 28*x*y^3 + 7*y^4)/4 + (6*x^5 + 30*x^4*y + 60*x^3*y^2 + 60*x^2*y^3 + 30*x*y^4 + 6*y^5)/5 + (12*x^6 + 72*x^5*y + 180*x^4*y^2 + 240*x^3*y^3 + 180*x^2*y^4 + 72*x*y^5 + 12*y^6)/6 + (8*x^7 + 56*x^6*y + 168*x^5*y^2 + 280*x^4*y^3 + 280*x^3*y^4 + 168*x^2*y^5 + 56*x*y^6 + 8*y^7)/7 + (15*x^8 + 120*x^7*y + 420*x^6*y^2 + 840*x^5*y^3 + 1050*x^4*y^4 + 840*x^3*y^5 + 420*x^2*y^6 + 120*x*y^7 + 15*y^8)/8 + ...
%e begins
%e n=0: [0, 1, 3, 4, 7, 6, 12, 8, 15, 13, 18, ..., sigma(k), ...];
%e n=1: [1, 6, 12, 28, 30, 72, 56, 120, 117, 180, ...];
%e n=2: [3, 12, 42, 60, 180, 168, 420, 468, 810, 660, ...];
%e n=3: [4, 28, 60, 240, 280, 840, 1092, 2160, 1980, 6160, ...];
%e n=4: [7, 30, 180, 280, 1050, 1638, 3780, 3960, 13860, 10010, ...];
%e n=5: [6, 72, 168, 840, 1638, 4536, 5544, 22176, 18018, 48048, ...];
%e n=6: [12, 56, 420, 1092, 3780, 5544, 25872, 24024, 72072, 120120, ...];
%e n=7: [8, 120, 468, 2160, 3960, 22176, 24024, 82368, 154440, 354640, ...];
%e n=8: [15, 117, 810, 1980, 13860, 18018, 72072, 154440, 398970, 437580, ...];
%e n=9: [13, 180, 660, 6160, 10010, 48048, 120120, 354640, 437580, 1896180, ...];
%e n=10: [18, 132, 1848, 4004, 24024, 72072, 248248, 350064, 1706562, 1847560, ...]; ...
%e in which the diagonal of coefficients of x^n*y^n/(2*n) equals
%e [0, 6, 42, 240, 1050, 4536, 25872, 82368, 398970, 1896180, ..., 2*a(n), ...],
%e which is twice this sequence.
%o (PARI) {a(n) = sigma(2*n) * binomial(2*n,n)/2}
%o for(n=1, 30, print1( a(n), ", ") )
%o (PARI) /* [x^n*y^n/n] log( Product_{n>=1} 1/(1 - (x + y)^n) ) */
%o N=30
%o {L = sum(n=1, 2*N+1, -log(1 - (x + y)^n +x*O(x^(2*N)) +y*O(y^(2*N))) ); }
%o {a(n) = polcoeff( n*polcoeff( L, n, x), n, y)}
%o for(n=1, N, print1( a(n), ", ") )
%Y Cf. A322186, A322203, A322187.
%K nonn
%O 1,1
%A _Paul D. Hanna_, Dec 07 2018
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