Unique properties of ‘37’ and ‘73’

‘37’ is the 12th prime number and ‘73’ is the 21st prime number. ‘73’ is the mirror image of 37 and 21 is the mirror image of 12. Moreover, the factors of 21 are 3 and 7, the digits which constitute 37 and 73.

73 is an ‘emirp’ (prime spelled backwards). When we reverse the digits of most primes, we get a composite (for example, 29 becomes 92). A palindrome prime reads the same forward and backwards, so reversing a palindrome prime gives us a prime (for example, 131). An emirp is a prime that gives us a different prime when its digits are reversed. 73 and 37 are mutually emirps.