pallindromes.rb

primes = (1..10000).to_a.delete_if {|n| ('1' * n) =~ /^1?$|^(11+?)\1+$/} 

bases = Hash.new
(2..20).each do |i|
  ary = []
  primes.each do |j|
    str = j.to_s(i)
    mid = str.length / 2
    p1 = str[0..mid-1]
    mid = str.length % 2 > 0 ? mid + 1 : mid
    p2 = str[mid..-1]
    ary.push(j) if(p1 == p2.reverse)
  end
  bases[i] = ary
end

bases.sort {|a,b| b[1].length <=> a[1].length} .each do |k,v|
  puts "#{k}: #{v.length}"
end