

A062721


Numbers k such that k is a product of two primes and k2 is prime.


10



4, 9, 15, 21, 25, 33, 39, 49, 55, 69, 85, 91, 111, 115, 129, 133, 141, 159, 169, 183, 201, 213, 235, 253, 259, 265, 295, 309, 319, 339, 355, 361, 381, 391, 403, 411, 445, 451, 469, 481, 489, 493, 501, 505, 511, 543, 559, 565, 573, 579, 589, 633, 649, 655, 679
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OFFSET

1,1


COMMENTS

This sequence is a subsequence of A107986, which only requires k to be composite. The first term in that sequence which is not in this sequence is 45, a number with three prime factors.  Alonso del Arte, May 03 2014


LINKS

Zak Seidov, Table of n, a(n) for n = 1..10000.


MATHEMATICA

Select[ Range[ 2, 1500 ], Plus @@ Last@Transpose@FactorInteger[ # ] == 2 && PrimeQ[ #  2 ] & ]
Select[Range[700], PrimeOmega[#] == 2 && PrimeQ[#  2]&] (* Harvey P. Dale, Mar 25 2013 *)


PROG

(PARI) { n=0; for (m=1, 10^9, a=prime(m) + 2; f=factor(a)~; if ((length(f)==1 && f[2, 1]==2)  (length(f)==2 && f[2, 1]==1 && f[2, 2]==1), write("b062721.txt", n++, " ", a); if (n==10000, break)) ) } \\ Harry J. Smith, Aug 09 2009


CROSSREFS

Cf. A006512, A001358, A043326.
Cf. A063637, A046315, A089268.
Sequence in context: A313278 A313279 A107986 * A104243 A313280 A313281
Adjacent sequences: A062718 A062719 A062720 * A062722 A062723 A062724


KEYWORD

nonn,easy


AUTHOR

Ahmed Fares (ahmedfares(AT)mydeja.com), Jul 14 2001


STATUS

approved



