OFFSET
1,1
EXAMPLE
2 is a term because sigma(2) = 3. 3! - 1 = 5, a prime.
6 is a term because sigma(6) = 12. 12! - 1 = 479001599, a prime.
PROG
(SageMath)
[n for n in range(1, 150) if is_prime(factorial(sigma(n))-1)]
(PARI) isok(n) = isprime(sigma(n)!-1); \\ Michel Marcus, Aug 07 2019
CROSSREFS
KEYWORD
nonn,more
AUTHOR
Hauke Löffler, Aug 07 2019
EXTENSIONS
a(12)-a(24) from Daniel Suteu, Aug 07 2019
a(25)-a(31) from Amiram Eldar, May 14 2023
STATUS
approved