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

1,2

Links

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

Example

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

Maple

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

Prog

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

Crossrefs

Cf. A000005, A000010, A049559.

Sequence in context: A045802 A006133 A317860 * A072849 A287930 A287799

Adjacent sequences: A039763 A039764 A039765 * A039767 A039768 A039769

Keyword

nonn,easy

Author

Olivier Gérard

Extensions

Term 1 prepended by Michel Marcus, May 30 2014

Status

approved