A057826 - OEIS

A057826

Greatest number with totient 2n (or zero when no such number exists).

6, 12, 18, 30, 22, 42, 0, 60, 54, 66, 46, 90, 0, 58, 62, 120, 0, 126, 0, 150, 98, 138, 94, 210, 0, 106, 162, 174, 118, 198, 0, 240, 134, 0, 142, 270, 0, 0, 158, 330, 166, 294, 0, 276, 0, 282, 0, 420, 0, 250, 206, 318, 214, 378, 242, 348, 0, 354, 0, 462, 0, 0, 254, 510

(list; graph; refs; listen; history; text; internal format)

OFFSET

1,1

COMMENTS

If a(n) = 0, n is a nontotient number - see (A005277)/2.

LINKS

T. D. Noe, Table of n, a(n) for n=1..10000

Max Alekseyev, PARI/GP Scripts for Miscellaneous Math Problems (invphi.gp).

MATHEMATICA

a = Table[0, {100}]; Do[ t = EulerPhi[n]/2; If[t < 101, a[[t]] = n], {n, 1, 10^3}]; a

PROG

(PARI) a(n) = invphiMax(2*n); \\ Amiram Eldar, Nov 11 2024, using Max Alekseyev's invphi.gp