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A107890
Semiprimes that are the product of two members of A007645.
2
9, 21, 39, 49, 57, 91, 93, 111, 129, 133, 169, 183, 201, 217, 219, 237, 247, 259, 291, 301, 309, 327, 361, 381, 403, 417, 427, 453, 469, 471, 481, 489, 511, 543, 553, 559, 579, 589, 597, 633, 669, 679, 687, 703, 721, 723, 763, 793, 813, 817, 831, 849, 871
OFFSET
1,1
REFERENCES
Conway, J. H. and Guy, R. K., The Book of Numbers. New York: Springer-Verlag, pp. 220-223, 1996.
Wagon, S. "Eisenstein Primes." Section 9.8 in Mathematica in Action. New York: W. H. Freeman, pp. 319-323, 1991.
LINKS
Eric Weisstein's World of Mathematics, Eisenstein Integer.
Eric Weisstein's World of Mathematics, Eisenstein Prime.
Eric Weisstein's World of Mathematics, Semiprime.
FORMULA
{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}.
MAPLE
N:= 1000: # for terms <= N
P:= [3, op(select(isprime, [seq(i, i=1..N/3, 6)]))]:
R:= NULL:
for i from 1 while P[i]^2 <= N do
m:= ListTools:-BinaryPlace(P, N/P[i]+1/2);
R:= R, seq(P[i]*P[j], j=i..m);
od:
sort([R]); # Robert Israel, Aug 28 2020
CROSSREFS
KEYWORD
easy,nonn
AUTHOR
Jonathan Vos Post, Jun 12 2005
EXTENSIONS
Edited by Ray Chandler, Oct 15 2005
Definition corrected by N. J. A. Sloane, Feb 06 2008
STATUS
approved