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A336115
Semiprimes that are product of a Fermat prime and a Mersenne prime.
1
9, 15, 21, 35, 51, 93, 119, 155, 381, 527, 635, 771, 1799, 2159, 7967, 24573, 32639, 40955, 139247, 196611, 393213, 458759, 655355, 1572861, 2031647, 2105087, 2228207, 2621435, 8323199, 8912879, 33685247, 134741759, 536813567, 6442450941, 8590000127, 10737418235
OFFSET
1,1
COMMENTS
As 3 is both a Fermat prime and a Mersenne prime, A019434(1) * A000668(1) = 9 is also a term. It is the only square in this sequence.
FORMULA
A335885(a(n)) = 2.
PROG
(PARI)
isA000668(n) = (isprime(n)&&!bitand(n, 1+n));
isA019434(n) = ((n>2)&&isprime(n)&&!bitand(n-2, n-1));
isA336115(n) = if(9==n, 1, if((2!=omega(n))||(2!=bigomega(n)), 0, my(ps=factor(n)[, 1]); (isA019434(ps[1])&&isA000668(ps[2]))||(isA019434(ps[2])&&isA000668(ps[1])))); \\ Antti Karttunen, Jul 11 2020
CROSSREFS
Subsequence of A335912.
Sequence in context: A241809 A063174 A338082 * A072569 A072572 A029708
KEYWORD
nonn
AUTHOR
Antti Karttunen, Jul 09 2020
EXTENSIONS
Missing terms and more terms added by Jinyuan Wang, Jul 11 2020
STATUS
approved