A367947 Semiprimes s such that 2*s+1 is prime.

6, 9, 14, 15, 21, 26, 33, 35, 39, 51, 65, 69, 74, 86, 95, 111, 119, 134, 141, 146, 155, 158, 183, 194, 209, 215, 219, 221, 249, 254, 278, 299, 303, 309, 321, 323, 326, 329, 341, 371, 386, 393, 398, 411, 413, 453, 473, 485, 515, 519, 543, 545, 551, 554, 581, 611, 614

Offset: 1

Author

Alexandre Herrera, Dec 05 2023

Keywords

• nonn

Crossrefs

Intersection of A001358 and A005097.

Cf. A063640.

Programs

PARI

isok(s) = (bigomega(s)==2) && isprime(2*s+1); \\ Michel Marcus, Dec 06 2023

Python

import sympy as sp
l = []
for i in range(620):
    if (sum(sp.factorint(i).values()) == 2) and sp.isprime(2*i+1):
        l.append(i)
print(l)

Formula

a(n) = (A063640(n) - 1)/2. - Hugo Pfoertner, Dec 05 2023