OFFSET
1,1
COMMENTS
Primes p such that p-1 and p+1 each have at most one odd prime factor (counted with multiplicity).
Terms > 3 must be either of the form 3*2^k+1 with 3*2^(k-1)+1 prime, or of the form 3*2^k-1 with 3*2^(k-1)-1 prime.
There are no more terms up to 3*2^5000+1.
Conjecture: these are all the terms.
EXAMPLE
a(5) = 11 is a term because 11-1=5*2^1 and 11+1=3*2^2 with 11, 5 and 3 prime.
MAPLE
{3, 7} union select(t -> isprime(t) and isprime((t+1)/2), {seq(3*2^k+1, k=1..3000)})
union select(t -> isprime(t) and isprime((t-1)/2), {seq(3*2^k-1, k=1..3000)});
CROSSREFS
KEYWORD
nonn,more
AUTHOR
J. M. Bergot and Robert Israel, May 05 2021
STATUS
approved