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

1,2

Comments

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

Links

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

Formula

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

Example

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.

Mathematica

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

Crossrefs

Cf. A000040, A001358, A174798.

Sequence in context: A348621 A277794 A292126 * A328345 A371309 A220574

Adjacent sequences: A174794 A174795 A174796 * A174798 A174799 A174800

Keyword

nonn

Author

Juri-Stepan Gerasimov, Mar 29 2010

Extensions

Edited and corrected by Ray Chandler, Apr 07 2010

Status

approved