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

1,1

Comments

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

Links

Table of n, a(n) for n=1..54.

Prog

(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

Crossrefs

Sequence in context: A080286 A075381 A030572 * A094320 A200546 A304342

Adjacent sequences: A175677 A175678 A175679 * A175681 A175682 A175683

Keyword

nonn

Author

Juri-Stepan Gerasimov, Aug 08 2010

Extensions

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

Status

approved