OFFSET
1,1
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.
LINKS
Robert Price, Table of n, a(n) for n = 1..1366
FORMULA
a(n) = A001358(n) - 1.
EXAMPLE
The smallest semiprime is 4 = 2*2, so a(1) = 4-1 = 3.
MATHEMATICA
Select[Range[5000], 2 == Total[Transpose[FactorInteger[# + 1]][[2]]] &]
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Jonathan Vos Post, Feb 24 2011
STATUS
approved