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

Offset: 1

Author

Robert G. Wilson v, Nov 08 2000

Keywords

• nonn

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).

Crossrefs

Bisection of A057635. Cf. A000010, A005277, A002181.

Programs

Mathematica

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

PARI

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