A174797 Numbers n such that both semiprime(n) and semiprime(n+1) are even.

1, 4, 21, 87, 120, 141, 142, 168, 188, 320, 363, 387, 434, 464, 496, 539, 593, 675, 721, 794, 810, 995, 1032, 1105, 1147, 1166, 1228, 1356, 1391, 1477, 1478, 1498, 1702, 1771, 1790, 1797, 1803, 1818, 1849, 1850, 1881, 1967, 2016, 2039, 2052, 2065, 2091

Offset: 1

Author

Juri-Stepan Gerasimov, Mar 29 2010

Keywords

• nonn

Comments

Semiprime(n)/2 and semiprime(n+1)/2 are consecutive primes.

Examples

			a(1)=1 because both semiprime(1)=4 and semiprime(2)=6 are even
a(2)=4 because both semiprime(4)=10 and semiprime(5)=14 are even.
		

Links

• Vincenzo Librandi, Table of n, a(n) for n = 1..2000

Crossrefs

Cf. A000040, A001358, A174798.

Programs

Mathematica

Flatten[Position[Partition[Select[Range[10000],PrimeOmega[#]==2&],2,1],?(EvenQ[First[#]]&&EvenQ[Last[#]]&),{1},Heads->False]] (* _Harvey P. Dale, Apr 19 2014 *)

Formula

A001358(a(n)) = 2*A000040(A174798(n)).

Extensions

Edited and corrected by Ray Chandler, Apr 07 2010