A217837 Numbers n such that 4n - 1 and 4n + 1 are both semiprimes.

14, 23, 30, 36, 40, 46, 51, 54, 55, 59, 62, 72, 75, 76, 80, 82, 85, 98, 103, 104, 113, 118, 129, 132, 134, 136, 138, 145, 167, 170, 172, 174, 195, 198, 204, 225, 240, 264, 270, 275, 280, 284, 285, 287, 292, 301, 314, 318, 329, 336, 337, 339, 346, 347, 348

Offset

1,1

Comments

This is to semiprimes as A045753 is to primes.

Links

Zak Seidov, Table of n, a(n) for n = 1..1000

Example

a(1) = 14 because 56 - 1 = 5*11 and 56 + 1 = 3*19 are semiprimes, and this is the first number having this property.
a(2) = 23 because 92 - 1 = 7*13 and 92 + 1 = 3*31 are semiprimes, and this is the second number having this property.
a(3) = 30 because 120 - 1 = 7*17 and 120 + 1 = 11*11 are semiprimes, and this is the third number having this property.

Mathematica

Select[Range[500], PrimeOmega[4# - 1] == 2 && PrimeOmega[4# + 1] == 2 &] (* Alonso del Arte, Oct 12 2012 *)

Crossrefs

Cf. A045753.

Sequence in context: A133150 A005734 A003694 * A285248 A102876 A188166

Adjacent sequences: A217834 A217835 A217836 * A217838 A217839 A217840

Keyword

nonn

Author

Zak Seidov, Oct 12 2012

Status

approved