A245015 Numbers n such that sigma(n) is a factorial.

1, 5, 14, 15, 23, 54, 56, 87, 95, 264, 270, 280, 354, 376, 406, 418, 435, 459, 478, 537, 623, 649, 667, 719, 1560, 1740, 1824, 1836, 1992, 2148, 2214, 2262, 2296, 2392, 2470, 2492, 2514, 2528, 2596, 2668, 2876, 3058, 3154, 3471, 3567, 3777, 3835, 3895, 4147, 4195, 4199

Offset: 1

Author

Alex Ratushnyak, Sep 16 2014

Keywords

• nonn

Comments

Numbers n such that A000203(n) is in A000142.

Examples

			sigma(5) is 6 which is 3!, so 5 is n the sequence.
		

Links

• Jens Kruse Andersen, Table of n, a(n) for n = 1..10000

Crossrefs

Cf. A000203, A000142, A055486.

Programs

Mathematica

Select[Range[4000], MemberQ[Range[8]!, DivisorSigma[1, #]] &] (* Alonso del Arte, Sep 24 2014 *)

PARI

isok(n) = {if (n==1, return (1)); s = sigma(n); f = 1; for (k=2, s, f *= k; if (f == s, return (1)); if (f > s, return (0)););} \\ Michel Marcus, Mar 09 2015