cp's OEIS Frontend

A068640 Define f(n) = 2n+1, a(n) = largest prime of the form f(f(f(...(n))). If no such prime exists then a(n) = 0.

%I A068640 #7 Dec 05 2013 19:55:11
%S A068640 7,47,7,0,47,13,0,17,19,0,47,0,0,59,31,0,0,37,0,167,43,0,47,0,0,107,0,
%T A068640 0,59,61,0,0,67,0,71,73,0,0,79,0,167,0,0,2879,0,0,0,97,0,101,103,0,
%U A068640 107,109,0,227,0,0,0,0,0,0,127,0,263,0,0,137,139,0,0,0,0,149,151,0,0,157,0
%N A068640 Define f(n) = 2n+1, a(n) = largest prime of the form f(f(f(...(n))). If no such prime exists then a(n) = 0.
%e A068640 a(2) = 47 as f(2) = 5, f(5) = 11,f(11) = 23, f(23) = 47 is the largest such prime . f(47) = 95 is not a prime. a(4) = 0 as f(4) = 9 is composite.
%p A068640 for k from 1 to 500 do a := 2*k+1; while(isprime(a)) do a := 2*a+1; end do; c[k] := (a-1)/2; if(not isprime(c[k])) then c[k] := 0; end if; if(c[k]<2*k+1) then c[k] := 0; end if; end do:q := seq(c[i],i=1..500);
%Y A068640 Cf. A068638.
%K A068640 nonn
%O A068640 1,1
%A A068640 _Amarnath Murthy_, Feb 27 2002
%E A068640 More terms from _Sascha Kurz_, Mar 17 2002