#!/bin/ksh

# Prime numbers for which the sum of their decimal digits are also primes

#  # Variables:
#
integer MAX_n=500

#  # Functions:
#
#  # Function _isprime(n) return 1 for prime, 0 for not prime
#
function _isprime {
   typeset _n ; integer _n=$1
   typeset _i ; integer _i

   (( _n < 2 )) && return 0
   for (( _i=2 ; _i*_i<=_n ; _i++ )); do
      (( ! ( _n % _i ) )) && return 0
   done
   return 1
}

#  # Function _sumdigits(n) return sum of n's digits
#
function _sumdigits {
   typeset _n ; _n=$1
   typeset _i _sum ; integer _i _sum=0

   for ((_i=0; _i<${#_n}; _i++)); do
      (( _sum+=${_n:${_i}:1} ))
   done
   echo ${_sum}
}

 ######
# main #
 ######

integer i digsum
for ((i=2; i<MAX_n; i++)); do
   _isprime ${i} && (( ! $? )) && continue

   digsum=$(_sumdigits ${i})
   _isprime ${digsum} ; (( $? )) && printf "%4d " ${i}
done
print