A039766 Numbers k such that gcd(phi(k), k-1) = number of divisors of k.

1, 3, 21, 33, 57, 69, 77, 85, 93, 105, 125, 129, 141, 161, 175, 177, 201, 205, 209, 213, 221, 237, 249, 253, 309, 321, 329, 345, 365, 381, 393, 413, 417, 437, 445, 453, 473, 475, 485, 489, 493, 497, 501, 517, 533, 537, 565, 573, 581, 597, 629, 633, 649, 665

Offset: 1

Author

Olivier Gérard

Keywords

• nonn
• easy

Examples

			phi(21)=12, gcd(12,20)=4, 21 is divisible by {1,3,7,21}.
		

Links

• Robert Israel, Table of n, a(n) for n = 1..10000

Crossrefs

Cf. A000005, A000010, A049559.

Programs

Maple

filter:= t -> igcd(numtheory:-phi(t),t-1) = numtheory:-tau(t):
select(filter,[$1..1000]); # Robert Israel, Mar 15 2019

PARI

isok(n) = gcd(eulerphi(n), n-1) == numdiv(n); \\ Michel Marcus, May 30 2014

Extensions

Term 1 prepended by Michel Marcus, May 30 2014