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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 (list; graph; refs; listen; history; text; internal format)
 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 Robert Israel, Table of n, a(n) for n = 1..10000 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 Cf. A001358, A007645, A108164. Sequence in context: A059993 A036704 A338911 * A110209 A184040 A242990 Adjacent sequences: A107887 A107888 A107889 * A107891 A107892 A107893 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

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Last modified March 30 07:09 EDT 2023. Contains 361606 sequences. (Running on oeis4.)