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 A103507 a(n) = Least i > 1, such that 2n+1 = 2*A000040(i)+A000040(k) for some k>1, 0 if no such i exists. 4
 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; 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=7, 2*7+1 = 15 = 2*5+5 and 5=A000040(3), thus a(7)=3. 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}] 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))))))) CROSSREFS a(n) = A049084(A103153(n)), for n >= 4. Can be used to compute A103153 and A103508. Cf. A103509. Sequence in context: A022922 A239141 A195352 * A219252 A290839 A085694 Adjacent sequences:  A103504 A103505 A103506 * A103508 A103509 A103510 KEYWORD nonn AUTHOR Lei Zhou, Feb 09 2005 EXTENSIONS Edited, Scheme-code added and starting offset changed from 0 to 1 by Antti Karttunen, Jun 19 2007 STATUS approved

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Last modified October 18 17:13 EDT 2019. Contains 328186 sequences. (Running on oeis4.)