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%I #24 Jul 31 2015 02:52:00
%S 9,21,33,49,57,69,77,93,121,129,133,141,161,177,201,209,213,217,237,
%T 249,253,301,309,321,329,341,361,381,393,413,417,437,453,469,473,489,
%U 497,501,517,529,537,553,573,581,589,597,633,649,669,681,713,717,721,737
%N Products of two primes of the form 4n+3 (A002145).
%C Every odd semiprime must be in one of three disjoint sets: the product of two primes of the form 4n+1 (A121387), the product of two primes of the form 4n+3 (A107978), or the product of a prime of the form 4n+1 and a prime of the form 4n+3 (A080774).
%H T. D. Noe, <a href="/A107978/b107978.txt">Table of n, a(n) for n=1..1000</a>
%H Eric Weisstein's World of Mathematics, <a href="http://mathworld.wolfram.com/Semiprime.html">Semiprime.</a>
%F {a(n)} = {p*q: p and q both elements of A002145}.
%t p = Select[ Prime@ Range@ 60, Mod[ #, 4] == 3 &]; Take[ Sort@ Flatten@ Table[ p[[i]] p[[j]], {j, 30}, {i, j}], 54] (* or *)
%t fQ[n_] := Block[{fi = FactorInteger@ n}, Plus @@ Last /@ fi == 2 && Union@ Mod[ First /@ fi, 4] == {3}]; Select[ Range@ 748, fQ@# &] (* _Robert G. Wilson v_, May 20 2010 *)
%Y Cf. A001358, A002145, A080774, A121387.
%Y Union of A131574 and A080109.
%Y Third row of A121388.
%K easy,nonn
%O 1,1
%A _Jonathan Vos Post_, Jun 12 2005
%E Edited by _N. J. A. Sloane_, May 20 2010
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