%N Total number of palindromic primes in base 3 below 3^n.
%C Every palindrome with an even number of digits is divisible by 11 (in base 3) and therefore is composite (not prime). Hence there is no palindromic prime with an even number of digits.
%H Eric Weisstein: <a href="http://mathworld.wolfram.com/PalindromicPrime.html">Palindromic Prime</a>.
%Y Cf. A029971, A117698.
%A _Martin Renner_, Apr 15 2006