A136196 Numbers n such that n and n+2 are consecutive semiprimes.

4, 49, 55, 91, 119, 143, 159, 183, 185, 203, 215, 219, 235, 247, 265, 287, 289, 299, 303, 319, 321, 327, 339, 391, 411, 413, 415, 451, 469, 471, 515, 517, 527, 533, 535, 543, 551, 579, 581, 589, 667, 669, 679, 685, 687, 695, 721, 753, 779, 789, 791, 813, 815

Offset: 1

Author

Zak Seidov, Dec 21 2007

Keywords

• nonn

Examples

			4 and 6 are 1st and 2nd semiprimes,
49 and 51 are 17th and 18th semiprimes,
55 and 57 are 19th and 20th semiprimes.
		

Links

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

Crossrefs

Cf. A001358, A065516.

Programs

Mathematica

Position[Partition[PrimeOmega[Range[900]],3,1],?(#[[1]]==#[[3]]==2 && #[[2]]!=2&),1,Heads->False]//Flatten (* _Harvey P. Dale, Oct 15 2021 *)

PARI

isok(n) = (bigomega(n) == 2) && (bigomega(n+2) == 2) && (bigomega(n+1) != 2); \\ Michel Marcus, Oct 13 2013