A186621 Semiprimes - 1.

3, 5, 8, 9, 13, 14, 20, 21, 24, 25, 32, 33, 34, 37, 38, 45, 48, 50, 54, 56, 57, 61, 64, 68, 73, 76, 81, 84, 85, 86, 90, 92, 93, 94, 105, 110, 114, 117, 118, 120, 121, 122, 128, 132, 133, 140, 141, 142, 144, 145, 154, 157, 158, 160, 165, 168, 176, 177, 182, 184, 186, 193

Offset: 1

Author

Jonathan Vos Post, Feb 24 2011

Keywords

• nonn
• easy

Comments

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.

Examples

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

Links

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

Crossrefs

Cf. A001358, A006093 Primes minus 1.

Programs

Mathematica

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

Formula

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