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 A103509 a(n) = Least i, such that 2n+1 = 2*A000040(k)+A000040(i) for some k>1, 0 if no such i exists. 4
 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 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,4 LINKS 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=11, 2*11+1 = 23 = 13+2*5 and 13=A000040(6), thus a(11)=6. MATHEMATICA 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}] PROG (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)))))))) CROSSREFS 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 KEYWORD nonn AUTHOR Lei Zhou, Feb 10 2005 EXTENSIONS Edited and Scheme-code added by Antti Karttunen, Jun 19 2007 STATUS approved

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Last modified June 20 07:54 EDT 2013. Contains 226422 sequences.