login
A280405
Odd semiprimes that cannot be represented as 2p+3q, where p and q are primes.
2
9, 33, 51, 69, 87, 111, 123, 141, 159, 177, 201, 213, 237, 249, 267, 291, 303, 321, 339, 381, 393, 411, 447, 471, 489, 501, 519, 537, 573, 591, 633, 669, 681, 699, 717, 753, 771, 789, 807, 831, 843, 879, 921, 933, 951, 993
OFFSET
1,1
LINKS
EXAMPLE
33 = 3*11 is a semiprime, and cannot be represented as twice a prime plus three times a prime. 21=3*7 is a semiprime which CAN be represented in that form, i.e. 2*3+3*5, and thus is not in this sequence.
MAPLE
N:= 10^4: # to get all terms <= N
Primes:= select(isprime, [2, seq(i, i=3..N/2, 2)]):
Cands:= {seq(i, i=1..N, 2)} minus {seq(seq(2*p+3*q, p=Primes), q=Primes)}:
sort(convert(select(numtheory:-bigomega=2, Cands), list)); # Robert Israel, Jan 09 2017
CROSSREFS
Cf. A046315 (odd semiprimes)
Cf. A280389 (odd semiprimes which can be represented as 2p+3q, where p and q are prime)
Sequence in context: A111045 A020226 A175440 * A111351 A065064 A337234
KEYWORD
nonn
AUTHOR
Randy L. Ekl, Jan 02 2017
STATUS
approved