OFFSET
1,1
COMMENTS
Seems to be the same as "Even numbers with prime number of divisors" - Jason Earls, Jul 04 2001
Except for the first term, smallest number == 1 (mod prime(n)) having n divisors (by Fermat's little theorem). - Amarnath Murthy and Meenakshi Srikanth (menakan_s(AT)yahoo.com), Jun 20 2003
LINKS
Karl V. Keller, Jr., Table of n, a(n) for n = 1..460
FORMULA
a(n) = 2^(prime(n)-1) = 2^A006093(n).
a(n) = A005179(prime(n)). - R. J. Mathar, Aug 09 2019
Sum_{n>=1} 1/a(n) = A119523. - Amiram Eldar, Aug 11 2020
MATHEMATICA
Table[2^(p-1), {p, Table[Prime[n], {n, 1, 18}]}] (* Geoffrey Critzer, May 26 2013 *)
PROG
(PARI) forstep(n=2, 100000000, 2, x=numdiv(n); if(isprime(x), print(n)))
(PARI) a(n)=2^(prime(n)-1) \\ Charles R Greathouse IV, Apr 08 2012
(Python)
from sympy import isprime, divisor_count as tau
[2] + [2**(2*n) for n in range(1, 33) if isprime(tau(2**(2*n)))] # Karl V. Keller, Jr., Jul 10 2020
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Labos Elemer, May 22 2001
STATUS
approved