login
A039766
Numbers k such that gcd(phi(k), k-1) = number of divisors of k.
2
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
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
KEYWORD
nonn,easy
EXTENSIONS
Term 1 prepended by Michel Marcus, May 30 2014
STATUS
approved