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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
Zak Seidov, Dec 21 2007