A114231 Smallest number m such that prime(n) + 2*prime(n-m) is a prime.

1, 1, 1, 3, 3, 1, 1, 1, 2, 1, 3, 2, 4, 4, 2, 9, 1, 3, 2, 4, 5, 1, 5, 2, 8, 3, 1, 3, 1, 1, 3, 8, 2, 6, 1, 4, 3, 8, 2, 7, 7, 14, 9, 7, 1, 4, 3, 1, 1, 1, 5, 1, 1, 2, 8, 4, 1, 8, 2, 4, 1, 8, 3, 9, 5, 3, 2, 1, 4, 1, 4, 4, 2, 3, 2, 4, 2, 12, 3, 1, 1, 3, 12, 2, 1, 2, 5, 5, 3, 3, 10, 4, 19, 1, 6, 4, 8, 7, 2, 5, 9, 2, 3

Offset: 2

Author

Lei Zhou, Nov 18 2005

Keywords

• easy
• nonn

Examples

			n=2, prime(2)+2*prime(2-1)=3+2*2=7 is prime, so a(2)=1;
n=3, prime(3)+2*prime(3-1)=5+2*3=11 is prime, so a(3)=1;
...
n=17, prime(17)+2*prime(17-9)=59+2*19=97 is prime, so a(17)=9.
		

Links

• Reinhard Zumkeller, Table of n, a(n) for n = 2..10000

Crossrefs

Cf. A114231, A114227, A114228.

Cf. A010051, A000040.

Programs

Haskell

a114231 n = head [m | m <- [1..],
                      a010051 (a000040 n + 2 * a000040 (n - m)) == 1]
-- Reinhard Zumkeller, Oct 31 2013

Mathematica

Table[p1 = Prime[n1]; n2 = n1 - 1; p2 = Prime[n2]; While[cp = p1 + 2*p2; ! PrimeQ[cp], n2--; If[n2 == 0, Print[n1]]; p2 = Prime[n2]]; n1 - n2, {n1, 2, 201}]

Extensions

Edited definition to conform to OEIS style. - Reinhard Zumkeller, Oct 31 2013