A109289 Composite numbers which are not of the form p*q + 1, where p and q are distinct primes.

4, 6, 8, 9, 10, 12, 14, 18, 20, 21, 24, 25, 26, 28, 30, 32, 33, 38, 42, 44, 45, 46, 48, 49, 50, 51, 54, 55, 57, 60, 62, 64, 65, 68, 69, 72, 74, 76, 77, 80, 81, 82, 84, 85, 90, 91, 93, 98, 99, 100, 102, 104, 105, 106, 108, 110, 111, 114, 115, 117, 118, 121, 122, 125, 126, 128

Offset

1,1

Links

Table of n, a(n) for n=1..66.

Mathematica

Select[Range[2, 130], ! (PrimeQ[ # ] || (Plus @@ Last /@ FactorInteger[ # - 1] == 2 && Length[FactorInteger[ # - 1]] == 2)) &] (* Ray Chandler, Aug 25 2005 *)
fQ[n_] := Last /@ FactorInteger[n] != {1, 1}; Select[ Range[2, 128], !PrimeQ[ # ] && fQ[ # - 1] &] (* Robert G. Wilson v *)

Crossrefs

Cf. A109287, A109288, A109290.

Sequence in context: A227180 A129930 A128510 * A066071 A306307 A355170

Adjacent sequences: A109286 A109287 A109288 * A109290 A109291 A109292

Keyword

easy,nonn

Author

Giovanni Teofilatto, Aug 20 2005

Extensions

Extended by Robert G. Wilson v and Ray Chandler, Aug 25 2005

Status

approved