1,1

Numbers of the form k-1 where k is semiprime (or biprime), namely a product of exactly two (not necessarily distinct) primes. Used as relative base sequence for analogies to sequences involving number of form (p-1) for prime p.

Robert Price, Table of n, a(n) for n = 1..1366

a(n) = A001358(n) - 1.

The smallest semiprime is 4 = 2*2, so a(1) = 4-1 = 3.

Select[Range[5000], 2 == Total[Transpose[FactorInteger[# + 1]][[2]]] &]

Cf. A001358, A006093 Primes minus 1.

Sequence in context: A319828 A325540 A102529 * A002159 A050094 A137319

Adjacent sequences: A186618 A186619 A186620 * A186622 A186623 A186624

nonn,easy

Jonathan Vos Post, Feb 24 2011

approved