A217606 - OEIS

%I #9 Oct 10 2012 15:34:52

%S 13,61,73,181,37,97,109,193,229,157,241,313,349,277,337,397,421,373,

%T 541,433,409,457,757,661,577,709,613,601,853,769,733,1021,997,877,829,

%U 673,1033,1009,1069,1117,1129,937,1201,1297,1549,1093,1153,1249,1213,1381

%N a(n) is the least unused prime greater than 3 such that (a(n) + a(n-1))/2 is prime, with a(0)=13.

%C Conjecture: every prime of the form 12k+1 is a member.

%p a:=5:

%p l:=13:

%p L:=[l]:

%p while l < 3400 do

%p if isprime((l+a)/2) then

%p if not(a in L) then

%p if not a mod 12 = 1 then

%p print(a);

%p break;

%p end if;

%p L:=[op(L),a]:

%p l:=a:

%p a:=5:

%p else

%p a:=nextprime(a):

%p end if;

%p else

%p a:=nextprime(a):

%p end if;

%p end do;

%p L;

%Y Cf. A086519.

%K nonn

%O 0,1

%A _Pedja Terzic_, Oct 08 2012