A114231 - OEIS

%I #5 Oct 31 2013 17:39:46

%S 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,

%T 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,

%U 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

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

%H Reinhard Zumkeller, <a href="/A114231/b114231.txt">Table of n, a(n) for n = 2..10000</a>

%e n=2, prime(2)+2*prime(2-1)=3+2*2=7 is prime, so a(2)=1;

%e n=3, prime(3)+2*prime(3-1)=5+2*3=11 is prime, so a(3)=1;

%e ...

%e n=17, prime(17)+2*prime(17-9)=59+2*19=97 is prime, so a(17)=9.

%t 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}]

%o (Haskell)

%o a114231 n = head [m | m <- [1..],

%o                       a010051 (a000040 n + 2 * a000040 (n - m)) == 1]

%o -- _Reinhard Zumkeller_, Oct 31 2013

%Y Cf. A114231, A114227, A114228.

%Y Cf. A010051, A000040.

%K easy,nonn

%O 2,4

%A _Lei Zhou_, Nov 18 2005

%E Edited definition to conform to OEIS style. - _Reinhard Zumkeller_, Oct 31 2013