A307390 Primes p such that 2*p-1 is not prime.
5, 11, 13, 17, 23, 29, 41, 43, 47, 53, 59, 61, 67, 71, 73, 83, 89, 101, 103, 107, 109, 113, 127, 131, 137, 149, 151, 163, 167, 173, 179, 181, 191, 193, 197, 223, 227, 233, 239, 241, 251, 257, 263, 269, 277, 281, 283, 293, 311, 313, 317, 347, 349, 353, 359, 373, 383, 389, 397, 401, 409, 419, 421
Offset: 1
Keywords
Examples
a(3) = 13 is in the sequence because 13 is prime but 2*13-1 = 25 is not.
Links
- Robert Israel, Table of n, a(n) for n = 1..10000
Programs
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Maple
select(t -> isprime(t) and not isprime(2*t-1), [seq(i,i=3..1000,2)]);
Formula
a(n) = A109274(n) + 1. - Bhavik Mehta, Aug 14 2024
Comments