A175634 Chen semiprimes: semiprimes m such that m+4 is either a prime or a semiprime.

6, 9, 10, 15, 21, 22, 25, 33, 34, 35, 39, 49, 51, 55, 57, 58, 65, 69, 82, 85, 87, 91, 93, 111, 115, 118, 119, 123, 129, 133, 141, 142, 145, 155, 159, 169, 177, 183, 187, 201, 202, 205, 209, 213, 214, 215, 217, 219, 235, 237, 247, 249, 253, 259, 265, 267, 274, 287

Offset: 1

Author

Juri-Stepan Gerasimov, Aug 01 2010

Keywords

• nonn

Crossrefs

Cf. A109611, A175612.

Programs

Python

from sympy.ntheory.factor_ import primeomega, isprime
def issemiprime(n): return primeomega(n) == 2
def ok(n): return issemiprime(n) and (issemiprime(n+4) or isprime(n+4))
print(list(filter(ok, range(1, 288)))) # Michael S. Branicky, Apr 14 2021

Extensions

Corrected (86 replaced by 85) by R. J. Mathar, Aug 03 2010