OFFSET
1,2
FORMULA
It appears that k is in the sequence iff k is prime or k is in {1, 21, 822857} (Verified to 3.1*10^6). It is true that if k is the product of two distinct primes, then k=21. - George J. Schaeffer (gschaeff(AT)andrew.cmu.edu), Apr 30 2005, R. J. Mathar, Jan 25 2017
The are no other nonprime terms below 10^11. - Amiram Eldar, Jan 09 2024
PROG
(PARI) is(n) = {my(f = factor(n)); n*numdiv(f) % sigma(f) == n - 1; } \\ Amiram Eldar, Jan 09 2024
CROSSREFS
KEYWORD
nonn
AUTHOR
George J. Schaeffer (gschaeff(AT)andrew.cmu.edu), Apr 29 2005
STATUS
approved