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 A081380 Numbers n such that the sets of prime factors (ignoring multiplicity) of A000203(n) = sigma(n) and of A001157(n) = sigma_2(n) are identical. 2
 1, 180, 1444, 12996, 23805, 36100, 52020, 60228, 64980, 68832, 95220, 301140, 324900, 344160, 481824, 1505700, 1718721, 1720800, 2275758, 2409120, 3755844, 6874884, 6879645, 7965153, 8593605, 11378790, 12045600, 15930306, 17405892 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 REFERENCES J.-M. De Koninck, Ces nombres qui nous fascinent, Entry 180, p. 56, Ellipses, Paris 2008. LINKS Donovan Johnson, Table of n, a(n) for n = 1..67 (terms < 2^31) EXAMPLE n = 1444 = 2^2*19^2, sigma(1444) = 2667 = 3*7*127, sigma_2(1444) = 2744343 = 3^2*7^4*127, common factor set = {3,7,127}. MATHEMATICA ffi[x_] := Flatten[FactorInteger[x]] lf[x_] := Length[FactorInteger[x]] ba[x_] := Table[Part[ffi[x], 2*w-1], {w, 1, lf[x]}] Do[s=ba[DivisorSigma[1, n]]; s5=ba[DivisorSigma[2, n]]; If[Equal[s, s5], Print[n]], {n, 1, 1000000}] PROG (PARI) is(n)=factor(sigma(n))[, 1]==factor(sigma(n, 2))[, 1] \\ Charles R Greathouse IV, Feb 19 2013 CROSSREFS Cf. A000203, A001157, A081377, A081378. Sequence in context: A251204 A225933 A225932 * A115184 A154672 A211556 Adjacent sequences: A081377 A081378 A081379 * A081381 A081382 A081383 KEYWORD nonn AUTHOR Labos Elemer, Mar 26 2003 EXTENSIONS More terms from Lekraj Beedassy, Jul 18 2008 a(16)-a(29) from Donovan Johnson, May 24 2009 STATUS approved

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Last modified June 16 19:52 EDT 2024. Contains 373432 sequences. (Running on oeis4.)