A123476 - OEIS

A123476

a(n) = (n!)^2/phi(n!), where phi is Euler's totient function.

1, 4, 18, 72, 450, 2700, 22050, 176400, 1587600, 15876000, 192099600, 2305195200, 32464832400, 454507653600, 6817614804000, 109081836864000, 1970290678356000, 35465232210408000, 711274934886516000

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OFFSET

1,2

COMMENTS

a(n) is a solution to the equation phi(x) = n!.

LINKS

T. D. Noe, Table of n, a(n) for n=1..100

P. Erdos and J. Lambek, Problem 4221, Amer. Math. Monthly, 55 (1948), 103.

MATHEMATICA

Table[(n!)^2/EulerPhi[n!], {n, 20}] (* Harvey P. Dale, Nov 29 2015 *)

PROG

(PARI) a(n) = (n!)^2/eulerphi(n!); \\ Michel Marcus, Jul 11 2018

CROSSREFS

Cf. A000010, A000142, A002202 (values taken by phi(x)).

Cf. A055487 (least m such that phi(m) = n!).

Sequence in context: A267450 A199524 A083693 * A343580 A027905 A037497

Adjacent sequences: A123473 A123474 A123475 * A123477 A123478 A123479

KEYWORD

nonn

AUTHOR

T. D. Noe, Sep 27 2006

STATUS

approved