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 A108640 a(n) = Product_{k=1..n} sigma_{n-k}(k), where sigma_m(k) = sum{j|k} j^m. 1
 1, 2, 6, 60, 1260, 239904, 123263712, 872883648000, 35330106763980000, 15502816844111220549120, 32196148399600498119169883520, 2560463149313858442381787649990400000, 717635502576022020068175045395317927056000000 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 LINKS G. C. Greubel, Table of n, a(n) for n = 1..44 EXAMPLE a(5) = 1^4 * (1^3 +2^3) * (1^2 +3^2) * (1^1 +2^1 +4^1) * (1^0 +5^0) = 1 * 9 * 10 * 7 * 2 = 1260. MAPLE with(numtheory): s:=proc(n, k) local div: div:=divisors(n): sum(div[j]^k, j=1..tau(n)) end: a:=n->product(s(i, n-i), i=1..n): seq(a(n), n=1..14); # Emeric Deutsch, Jul 13 2005 MATHEMATICA Table[Product[DivisorSigma[j, n-j], {j, 0, n-1}], {n, 30}] (* G. C. Greubel, Oct 18 2023 *) PROG (PARI) a(n) = prod(k=1, n, sigma(k, n-k)); \\ Michel Marcus, Aug 16 2019 (Magma) A108639:= func< n | (&*[DivisorSigma(j, n-j): j in [0..n-1]]) >; [A108639(n): n in [1..30]]; // G. C. Greubel, Oct 18 2023 (SageMath) def A108640(n): return product(sigma(n-j, j) for j in range(n)) [A108640(n) for n in range(1, 31)] # G. C. Greubel, Oct 18 2023 CROSSREFS Cf. A108639 (with sums). Sequence in context: A156451 A152617 A156472 * A084971 A224883 A001577 Adjacent sequences: A108637 A108638 A108639 * A108641 A108642 A108643 KEYWORD nonn AUTHOR Leroy Quet, Jul 06 2005 EXTENSIONS More terms from Emeric Deutsch, Jul 13 2005 STATUS approved

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Last modified December 9 20:47 EST 2023. Contains 367695 sequences. (Running on oeis4.)