OFFSET
1,1
COMMENTS
For k > 9, these are semiprimes k such that 2^(k+1) == 1 (mod k): semiprimes in A187787.
In this sequence, only 9 is a perfect square. - Jinyuan Wang, Mar 30 2019
PROG
(PARI) isok(k) = (bigomega(k)==2) && (Mod(2, k)^(k+1) == 1); \\ (for k > 9) Michel Marcus, Mar 29 2019
(Perl) use ntheory ":all"; forsemiprimes { print "$_\n" if powmod(2, vecsum(factor($_)), $_) == 1 } 4, 1e7; # Daniel Suteu, Mar 30 2019
CROSSREFS
KEYWORD
nonn
AUTHOR
Thomas Ordowski, Mar 29 2019
EXTENSIONS
a(7)-a(18) from Amiram Eldar, Mar 29 2019
a(19)-a(29) from Daniel Suteu, Mar 29 2019
STATUS
approved