A207526 Primes p such that p-2 or p+2 are semiprimes.

2, 7, 11, 13, 17, 19, 23, 31, 37, 41, 47, 53, 59, 67, 71, 79, 83, 89, 97, 109, 113, 127, 131, 139, 157, 163, 167, 179, 181, 199, 211, 223, 233, 239, 251, 257, 263, 269, 293, 307, 311, 317, 331, 337, 353, 359, 367, 373, 379, 383, 389, 397, 401, 409, 419, 439

Offset

1,1

Links

Vincenzo Librandi, Table of n, a(n) for n = 1..5000

Example

2+2=2^2(semiprime), 7+2=3^2(semiprime), 11-2=3^2(semiprime), 13+2=3*5(semiprime), 17-2=3*5(semiprime)

Mathematica

Select[Table[Prime[n], {n, 200}], (Last /@ FactorInteger[#-2] == {1, 1} || Last /@ FactorInteger[#-2] == {2}) || (Last /@ FactorInteger[#+2] == {1, 1} || Last /@ FactorInteger[#+2] == {2}) &]

Crossrefs

Cf. A063637, A063638, A063643.

Sequence in context: A173135 A020627 A043292 * A175442 A020597 A140563

Adjacent sequences: A207523 A207524 A207525 * A207527 A207528 A207529

Keyword

nonn

Author

Vladimir Joseph Stephan Orlovsky, Feb 18 2012

Status

approved