A175680 Semiprimes that are not Chen semiprimes A175634.

4, 14, 26, 38, 46, 62, 74, 77, 86, 94, 95, 106, 121, 122, 134, 143, 146, 158, 161, 166, 178, 185, 194, 203, 206, 218, 221, 226, 254, 262, 278, 302, 314, 321, 326, 329, 334, 339, 341, 346, 362, 365, 371, 381, 386, 395, 398, 422, 437, 446, 451, 458, 466, 471

Offset: 1

Author

Juri-Stepan Gerasimov, Aug 08 2010

Keywords

• nonn

Comments

Non-Chen semiprimes: semiprimes m such that m+4 is neither a prime or a semiprime.

Programs

Python

from sympy.ntheory.factor_ import primeomega, isprime
def issemiprime(n): return primeomega(n) == 2
def ok(n): return issemiprime(n) and not (issemiprime(n+4) or isprime(n+4))
print(list(filter(ok, range(1, 472)))) # Michael S. Branicky, Apr 14 2021

Extensions

Corrected (234 replaced by 254, 471 inserted) by R. J. Mathar, Aug 10 2010