OFFSET
1,1
COMMENTS
Semiprimes in arithmetic progression. All terms are odd, see also A056809.
LINKS
Harvey P. Dale, Table of n, a(n) for n = 1..1000
MATHEMATICA
PrimeFactorExponentsAdded[n_] := Plus @@ Flatten[Table[ #[[2]], {1}] & /@ FactorInteger[n]]; Select[ Range[ 1792], PrimeFactorExponentsAdded[ # ] == PrimeFactorExponentsAdded[ # + 2] == PrimeFactorExponentsAdded[ # + 4] == 2 &] (* Robert G. Wilson v, Feb 24 2004 *)
SequencePosition[Table[If[PrimeOmega[n]==2, 1, 0], {n, 2000}], {1, _, 1, _, 1}][[All, 1]] (* Requires Mathematica version 10 or later *) (* Harvey P. Dale, Feb 17 2020 *)
PROG
(Magma)IsSemiprime:=func< n| &+[ k[2]: k in Factorization(n) ] eq 2 >; [ n: n in [2..4300]|IsSemiprime(n) and IsSemiprime(n+2) and IsSemiprime(n+4)] // Vincenzo Librandi, Dec 16 2010
CROSSREFS
KEYWORD
easy,nonn
AUTHOR
Zak Seidov, Feb 22 2004
STATUS
approved