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A325653 a(n) = the product of numbers k such that sigma(k) = sigma(n). 3
1, 2, 3, 4, 5, 66, 7, 8, 9, 170, 66, 12, 13, 4830, 4830, 400, 170, 18, 19, 21320, 651, 22, 4830, 53808, 400, 21320, 27, 1092, 29, 274833900, 651, 32, 54285, 1802, 54285, 36, 37, 53808, 1092, 206480, 21320, 13835052, 43, 237380, 45, 274833900, 54285, 3600, 49 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

LINKS

Robert Israel, Table of n, a(n) for n = 1..10000

EXAMPLE

a(6) = 66 because sigma(6) = sigma(11) = 12; 6 * 11 = 66.

MAPLE

N:= 1000: # to get a(n) before the first n with sigma(n) > N

S:= map(numtheory:-sigma, [$1..N-1]):

m:=min(select(t -> S[t]>N, [$1..N-1]))-1:

seq(convert(select(s -> S[s]=S[n], [$1..S[n]-1]), `*`), n=1..m); # Robert Israel, Jul 04 2019

MATHEMATICA

a[n_] := Block[{s = DivisorSigma[1, n]}, Product[Which[s == DivisorSigma[1, k], k, True, 1], {k, s}]]; Array[a, 49] (* Giovanni Resta, Jul 03 2019 *)

PROG

(MAGMA) [&*[k: k in[1..10000] | SumOfDivisors(k) eq SumOfDivisors(n)]: n in [1..100]]

(PARI) a(n) = {my(s=sigma(n)); prod(k=1, s, if ((sigma(k)==s), k, 1)); } \\ Michel Marcus, May 12 2019

CROSSREFS

Cf. A000203, A275987, A325651.

See A070242 and A325652 for number and sum of such numbers k.

Sequence in context: A062939 A069881 A004856 * A037327 A075832 A037434

Adjacent sequences:  A325650 A325651 A325652 * A325654 A325655 A325656

KEYWORD

nonn

AUTHOR

Jaroslav Krizek, May 12 2019

STATUS

approved

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Last modified October 22 05:36 EDT 2021. Contains 348160 sequences. (Running on oeis4.)