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A111351
Semiprimes of the form 2*n + 1, where n is a square.
3
9, 33, 51, 129, 201, 289, 339, 393, 451, 579, 649, 723, 1059, 1569, 1923, 2049, 3043, 3873, 4609, 4803, 5001, 5619, 5833, 6051, 6499, 6729, 7201, 7939, 8193, 9249, 9523, 10083, 11553, 12169, 13123, 13449, 14113, 14451, 14793, 15843, 16201, 16563
OFFSET
1,1
LINKS
FORMULA
a(n) = 2*A111350(n) + 1.
MATHEMATICA
Select[2Range[100]^2 + 1, Plus @@ Last /@ FactorInteger[ # ] == 2 &] (* Ray Chandler, Nov 10 2005 *)
Select[2*Range[100]^2+1, PrimeOmega[#]==2&] (* Harvey P. Dale, May 07 2013 *)
PROG
(Magma) IsSemiprime:=func<t | &+[d[2]: d in Factorization(t)] eq 2>; [s: t in [2..100] | IsSemiprime(s) where s is 2*t^2 + 1]; // Bruno Berselli, Sep 22 2012
CROSSREFS
Sequence in context: A020226 A175440 A280405 * A065064 A337234 A075812
KEYWORD
nonn,easy
AUTHOR
Giovanni Teofilatto, Oct 29 2005
EXTENSIONS
Edited and extended by Ray Chandler, Nov 10 2005
Offset corrected by Arkadiusz Wesolowski, Aug 09 2011
STATUS
approved