A125721 - OEIS

A125721

a(n)=2*n!/d(n!); d(m)=A000005(m) is the number of divisors of m.

2, 2, 2, 3, 6, 15, 48, 168, 840, 4536, 26880, 147840, 1209600, 7862400, 67267200, 648648000, 7783776000, 66162096000, 871782912000, 8281937664000, 118562476032000, 1680623097753600, 23416681828700160, 269291841030051840, 5109094217170944000

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OFFSET

0,1

COMMENTS

a(3)=3 and a(5)=15 are the only odd numbers in this sequence.

REFERENCES

P. Erdos, solved by J. Fiedler, Elem. Math. 16 (1961), 42-44, Aufgabe 374.

LINKS

Table of n, a(n) for n=0..24.

FORMULA

a(n)=2*A000142(n)/A027423(n).

EXAMPLE

a(4)=2*4!/d(4!)=2*24/8=6.

MATHEMATICA

Table[(2n!)/DivisorSigma[0, n!], {n, 0, 25}] (* Harvey P. Dale, Jun 01 2014 *)

PROG

(PARI) a(n) = 2*n!/numdiv(n!); \\ Michel Marcus, Aug 26 2013

CROSSREFS

Cf. A000142, A027423.

Sequence in context: A143596 A342763 A091712 * A049798 A165198 A245526

Adjacent sequences: A125718 A125719 A125720 * A125722 A125723 A125724

KEYWORD

nonn

AUTHOR

Franz Vrabec, Feb 01 2007

EXTENSIONS

More terms from Michel Marcus, Aug 26 2013

STATUS

approved