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A107890 Semiprimes that are the product of two members of A007645. 2

%I #13 Aug 29 2020 01:54:59

%S 9,21,39,49,57,91,93,111,129,133,169,183,201,217,219,237,247,259,291,

%T 301,309,327,361,381,403,417,427,453,469,471,481,489,511,543,553,559,

%U 579,589,597,633,669,679,687,703,721,723,763,793,813,817,831,849,871

%N Semiprimes that are the product of two members of A007645.

%D Conway, J. H. and Guy, R. K., The Book of Numbers. New York: Springer-Verlag, pp. 220-223, 1996.

%D Wagon, S. "Eisenstein Primes." Section 9.8 in Mathematica in Action. New York: W. H. Freeman, pp. 319-323, 1991.

%H Robert Israel, <a href="/A107890/b107890.txt">Table of n, a(n) for n = 1..10000</a>

%H Eric Weisstein's World of Mathematics, <a href="http://mathworld.wolfram.com/EisensteinInteger.html">Eisenstein Integer.</a>

%H Eric Weisstein's World of Mathematics, <a href="http://mathworld.wolfram.com/EisensteinPrime.html">Eisenstein Prime.</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 A007645} = {p*q: p and q both of form 3*m^2 * n^2 for integers m, n}.

%p N:= 1000: # for terms <= N

%p P:= [3,op(select(isprime, [seq(i,i=1..N/3,6)]))]:

%p R:= NULL:

%p for i from 1 while P[i]^2 <= N do

%p m:= ListTools:-BinaryPlace(P,N/P[i]+1/2);

%p R:= R, seq(P[i]*P[j],j=i..m);

%p od:

%p sort([R]); # _Robert Israel_, Aug 28 2020

%Y Cf. A001358, A007645, A108164.

%K easy,nonn

%O 1,1

%A _Jonathan Vos Post_, Jun 12 2005

%E Edited by _Ray Chandler_, Oct 15 2005

%E Definition corrected by _N. J. A. Sloane_, Feb 06 2008

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Last modified March 28 09:04 EDT 2024. Contains 371240 sequences. (Running on oeis4.)