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

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

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