A280405 Odd semiprimes that cannot be represented as 2p+3q, where p and q are primes.
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
Keywords
Examples
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.
Links
- Robert Israel, Table of n, a(n) for n = 1..10000
Crossrefs
Programs
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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
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