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A330409
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Semiprimes of the form p(6p - 1).
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0
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22, 51, 145, 287, 1717, 2147, 3151, 5017, 11051, 13207, 16801, 20827, 26867, 63551, 68587, 71177, 76501, 96647, 112477, 147737, 159251, 232657, 237407, 308947, 314417, 342487, 433897, 480251, 587501, 602617, 722107, 772927, 834401, 861467, 879751, 907537, 945257, 1155887, 1177051
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OFFSET
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1,1
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LINKS
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Table of n, a(n) for n=1..39.
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FORMULA
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a(n) = A049452(A158015(n)) = p(6p - 1) with p = A158015(n).
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EXAMPLE
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A158015(1) = 2 is the smallest prime p such that 6p - 1 = 12 - 1 = 11 is also prime, whence a(1) = A049452(2) = 2*(6*2 - 1) = 22.
prime(5) = 11 is the smallest prime not in A024898 or A158015, because 6p - 1 is not a prime, therefore A049452(11) = 11*(6*11 - 1) is not in the sequence, and idem for A049452(13).
But prime(7) = 17 is in A024898 and A158015, so a(5) = A024898(A158015(5)) = A024898(17) = 17*(6*17 - 1).
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PROG
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(PARI) [p*(6*p-1) | p<-primes(99), isprime(6*p-1)]
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CROSSREFS
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Cf. A024898 (6n-1 is prime), A158015 (primes), A049452 = {n(6n-1)}.
Complement of A255584 = A033570(A130800) (semiprimes (2n+1)(3n+1)) in A245365 (primes of the form n(3n-1)/2).
Sequence in context: A092221 A191279 A200880 * A111576 A277979 A177726
Adjacent sequences: A330406 A330407 A330408 * A330410 A330411 A330412
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KEYWORD
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nonn
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AUTHOR
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M. F. Hasler, Dec 13 2019
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STATUS
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approved
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