1,1

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

4 and 6 are 1st and 2nd semiprimes,

49 and 51 are 17th and 18th semiprimes,

55 and 57 are 19th and 20th semiprimes.

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

Cf. A001358, A065516.

Sequence in context: A189347 A248558 A198384 * A222960 A061100 A147803

Adjacent sequences: A136193 A136194 A136195 * A136197 A136198 A136199

nonn

Zak Seidov, Dec 21 2007

approved