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%I #5 Mar 31 2012 13:21:09
%S 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,
%T 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,
%U 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
%N a(n) = Least i > 1, such that 2n+1 = 2*A000040(i)+A000040(k) for some k>1, 0 if no such i exists.
%e 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.
%t 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}]
%o (Scheme, with Aubrey Jaffer's SLIB Scheme library from http://www.swiss.ai.mit.edu/~jaffer/SLIB.html )
%o (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)))))))
%Y a(n) = A049084(A103153(n)), for n >= 4. Can be used to compute A103153 and A103508. Cf. A103509.
%K nonn
%O 1,4
%A _Lei Zhou_, Feb 09 2005
%E Edited, Scheme-code added and starting offset changed from 0 to 1 by _Antti Karttunen_, Jun 19 2007
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