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A103509
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a(n) = Least i, such that 2n+1 = 2*A000040(k)+A000040(i) for some k>1, 0 if no such i exists.
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4
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0, 0, 0, 2, 3, 2, 3, 2, 3, 4, 6, 2, 3, 2, 3, 4, 6, 2, 3, 2, 3, 4, 6, 2, 3, 4, 7, 5, 6, 2, 3, 2, 3, 4, 6, 5, 6, 2, 3, 4, 12, 2, 3, 2, 3, 4, 6, 2, 3, 4, 7, 5, 6, 2, 3, 4, 10, 5, 6, 2, 3, 2, 3, 4, 6, 5, 6, 2, 3, 4, 12, 2, 3, 2, 3, 4, 6, 5, 6, 2, 3, 4, 18, 2, 3, 4, 7, 5, 6, 2, 3, 4, 10, 5, 6, 15, 7, 2, 3, 4, 12, 2, 3, 2, 3
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OFFSET
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1,4
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LINKS
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Table of n, a(n) for n=1..105.
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EXAMPLE
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For n < 4 there are no such primes, thus a(1)-a(3)=0. For n=4, 2*4+1 = 9 = 2*3+3 and 3=A000040(2), thus a(4)=2. For n=11, 2*11+1 = 23 = 13+2*5 and 13=A000040(6), thus a(11)=6.
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MATHEMATICA
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Do[m = 3; While[ ! (PrimeQ[m] && (((n - m)/2) > 2) && PrimeQ[(n - m)/2]), m = m + 2]; k = PrimePi[m]; Print[k], {n, 9, 299, 2}]
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PROG
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(Scheme, with Aubrey Jaffer's SLIB Scheme library from http://www.swiss.ai.mit.edu/~jaffer/SLIB.html )
(define (A103509 n) (let ((o (+ (* 2 n) 1))) (let loop ((i 2)) (let ((p2 (A000040 i))) (cond ((> p2 (- o 6)) 0) ((prime? (/ (- o p2) 2)) i) (else (loop (+ 1 i))))))))
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CROSSREFS
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a(n) = A049084(A103506(n)), for n >= 4. Can be used to compute A103506 and A103510. Cf. A103507.
Sequence in context: A071995 A114108 A073820 * A069898 A007978 A096737
Adjacent sequences: A103506 A103507 A103508 * A103510 A103511 A103512
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KEYWORD
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nonn
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AUTHOR
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Lei Zhou, Feb 10 2005
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EXTENSIONS
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Edited and Scheme-code added by Antti Karttunen, Jun 19 2007
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STATUS
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approved
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