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A103507
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a(n) = Least i > 1, such that 2n+1 = 2*A000040(i)+A000040(k) for some k>1, 0 if no such i exists.
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4
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0, 0, 0, 2, 2, 2, 3, 2, 2, 3, 2, 2, 3, 2, 4, 3, 2, 2, 3, 3, 2, 4, 2, 2, 3, 2, 4, 3, 2, 4, 3, 2, 2, 3, 3, 2, 4, 2, 2, 3, 3, 2, 4, 2, 8, 3, 2, 4, 3, 5, 2, 5, 2, 2, 3, 2, 2, 3, 2, 4, 3, 5, 4, 5, 5, 2, 5, 2, 6, 3, 2, 2, 3, 3, 4, 4, 2, 2, 3, 3, 2, 4, 3, 2, 4, 2, 6, 3, 2, 4, 3, 2, 2, 3, 3, 4, 4, 2, 2, 3, 2, 2, 3, 3, 4, 4, 5, 2
(list; graph; refs; listen; history; internal format)
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OFFSET
| 1,4
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EXAMPLE
| 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=7, 2*7+1 = 15 = 2*5+5 and 5=A000040(3), thus a(7)=3.
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MATHEMATICA
| Do[m = 3; While[ ! (PrimeQ[m] && ((n - 2*m) > 2) && PrimeQ[n - 2*m]), m = m + 2]; k = PrimePi[m]; Print[k], {n, 9, 299, 2}]
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PROG
| (Scheme, with Aubrey Jaffer's SLIB Scheme library from http://www.swiss.ai.mit.edu/~jaffer/SLIB.html )
(define (A103507 n) (let loop ((i 2)) (let ((p1 (A000040 i))) (cond ((>= p1 n) 0) ((prime? (+ 1 (* 2 (- n p1)))) i) (else (loop (+ 1 i)))))))
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CROSSREFS
| a(n) = A049084(A103153(n)), for n >= 4. Can be used to compute A103153 and A103508. Cf. A103509.
Sequence in context: A173883 A022922 A195352 * A085694 A160493 A053760
Adjacent sequences: A103504 A103505 A103506 * A103508 A103509 A103510
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KEYWORD
| nonn
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AUTHOR
| Lei Zhou (lzhou5(AT)emory.edu), Feb 09 2005
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EXTENSIONS
| Edited, Scheme-code added and starting offset changed from 0 to 1 by Antti Karttunen (His-Firstname.His-Surname(AT)gmail.com), Jun 19 2007
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