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A344103 a(n) is the smallest number k >= 1 with exactly n divisors d, for which sigma(k) is divisible by d*sigma(d). 1
1, 10, 6, 30, 132, 546, 270, 120, 840, 672, 1560, 3960, 4320, 6048, 9120, 16380, 26208, 12180, 36540, 37380, 10920, 55692, 34440, 68040, 112140, 51480, 63840, 103320, 30240, 219960, 273000, 209160, 332640, 191520, 1136520, 393120, 594720, 1389960, 1239840 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

LINKS

Table of n, a(n) for n=1..39.

EXAMPLE

sigma(1) = 1 = 1*sigma(1).

sigma(10) = 18 = 18*(1*sigma(1)) = 3*(2*sigma(2)).

sigma(6) = 12 = 12*(1*sigma(1)) = 2*(2*sigma(2)) = 1*(3*sigma(3)).

sigma(30) = 72 = 72*(1*sigma(1)) = 11*(2*sigma(2)) = 6*(3*sigma(3)) = 1*(6*sigma(6)) .

MATHEMATICA

a[n_] := Module[{k = 1}, While[Count[Divisors[k], _?(Divisible[DivisorSigma[1, k], # * DivisorSigma[1, #]] &)] != n, k++]; k]; Array[a, 25] (* Amiram Eldar, May 12 2021 *)

PROG

(MAGMA) sd:=func<n, d| DivisorSigma(1, n) mod (d*DivisorSigma(1, d)) eq 0>; a:=[];  for n in [1..32] do k:=1; while #[d:d in Divisors(k)|sd(k, d)] ne n do k:=k+1; end while; Append(~a, k); end for; a;

(PARI) isok(k, n) = my(sk=sigma(k)); sumdiv(k, d, (sk % (d*sigma(d))) == 0) == n;

a(n) = my(k=1); while (!isok(k, n), k++); k; \\ Michel Marcus, May 12 2021

CROSSREFS

Cf. A000203, A339472, A344102.

Sequence in context: A033986 A049004 A104645 * A181107 A234974 A248593

Adjacent sequences:  A344100 A344101 A344102 * A344104 A344105 A344106

KEYWORD

nonn

AUTHOR

Marius A. Burtea, May 10 2021

STATUS

approved

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Last modified July 23 19:17 EDT 2021. Contains 346259 sequences. (Running on oeis4.)