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A073809
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Number of common divisors of sigma[1,n] and sigma[3,n].
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2
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1, 2, 3, 1, 4, 6, 4, 4, 1, 6, 6, 6, 4, 8, 8, 2, 6, 2, 6, 8, 6, 9, 8, 12, 1, 8, 8, 4, 8, 12, 6, 3, 10, 8, 10, 1, 4, 12, 8, 12, 8, 12, 6, 6, 4, 12, 10, 6, 2, 2, 12, 4, 8, 16, 12, 16, 10, 12, 12, 16, 4, 12, 4, 2, 12, 15, 6, 12, 12, 15, 12, 8, 4, 8, 3, 12, 12, 16, 10, 8, 3, 12, 12, 12, 12, 12, 16
(list; graph; refs; listen; history; internal format)
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OFFSET
| 1,2
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FORMULA
| a(n)=Card[Intersection[D[A000203(n)], D[A001158(n)]]].
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EXAMPLE
| n=10: sigma[1,10]=18, sigma[3,10]=1134; Intersection[{1,2,3,6,9,18},{1,2,3,6,7,9,14,18,21,27,42,54,63, 81,126,162,189,378,567,1134}]={1,2,3,6,9,18}, so a(10)=6.
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MATHEMATICA
| g1[x_] := Divisors[DivisorSigma[1, x]] g2[x_] := Divisors[DivisorSigma[3, x]] ncd[x_] := Length[Intersection[g1[x], g2[x]]] Table[ncd[w], {w, 1, 128}]
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CROSSREFS
| Cf. A000203, A001158, A073802, A073808.
Sequence in context: A046671 A178760 A129566 * A111776 A189187 A171083
Adjacent sequences: A073806 A073807 A073808 * A073810 A073811 A073812
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KEYWORD
| nonn
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AUTHOR
| Labos E. (labos(AT)ana.sote.hu), Aug 13 2002
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