cp's OEIS Frontend

A217606 a(n) is the least unused prime greater than 3 such that (a(n) + a(n-1))/2 is prime, with a(0)=13.

%I A217606 #9 Oct 10 2012 15:34:52
%S A217606 13,61,73,181,37,97,109,193,229,157,241,313,349,277,337,397,421,373,
%T A217606 541,433,409,457,757,661,577,709,613,601,853,769,733,1021,997,877,829,
%U A217606 673,1033,1009,1069,1117,1129,937,1201,1297,1549,1093,1153,1249,1213,1381
%N A217606 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 A217606 Conjecture: every prime of the form 12k+1 is a member.
%p A217606 a:=5:
%p A217606 l:=13:
%p A217606 L:=[l]:
%p A217606 while l < 3400 do
%p A217606 if isprime((l+a)/2) then
%p A217606 if not(a in L) then
%p A217606 if not a mod 12 = 1 then
%p A217606 print(a);
%p A217606 break;
%p A217606 end if;
%p A217606 L:=[op(L),a]:
%p A217606 l:=a:
%p A217606 a:=5:
%p A217606 else
%p A217606 a:=nextprime(a):
%p A217606 end if;
%p A217606 else
%p A217606 a:=nextprime(a):
%p A217606 end if;
%p A217606 end do;
%p A217606 L;
%Y A217606 Cf. A086519.
%K A217606 nonn
%O A217606 0,1
%A A217606 _Pedja Terzic_, Oct 08 2012