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

Offset: 1

Author

T. D. Noe, Sep 27 2006

Keywords

• nonn

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.

Crossrefs

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

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

Programs

Mathematica

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

PARI

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