If you have not noticed yet, it is up on MAE.
Dr. Sheldon Cooper "The best # is 73. Why? 73 is the 21st prime #. Its mirror (37) is the 12th and its mirror (21) is 7 times 3
In binary, 73 it's a palindrome, 1001001 backwards is 1001001." like racecar
From the 73rd episode of Big Bang Theory