Thirteen is the unluckiest of numbers – if you happen to be superstitious. This belief has a couple of historical roots. According to Biblical tradition, there were 13 people at Christ's Last Supper, and Christ was crucified on Friday 13th. Further back in time, Alexander the Great decided he wanted to be the thirteenth god alongside the 12 that already stood for each month of the year, so he had a thirteenth statue built on the place of his capital. His death shortly after gave the number a bad name. Many buildings don't have a floor labeled 13 and many hotels will have room numbered 12A instead of 13. There is even a name for a morbid fear of 13: triskaidekaphobia. Fresh disasters involving the number hardly help triskaidekaphobics overcome their affliction. The most notorious of these involved the Apollo 13 Moon mission, which was launched on April 11, 1970 (the sum of 4, 11 and 70 equals 85, the digital sum of which is 13), from Pad 39 (three times 13) at 13:13 local time, and suffered an explosion on April 13. (The astronauts did, however, make it home safely, which could be considered good luck.) There is always at least one Friday 13th in each year; in some years, there are two and rarely three (for example, 1998 and 2009). There were 13 original U.S. colonies (hence the 13 stripes on the American flag) and 13 signers of the Declaration of Independence. In Japan, the numbers 4 and 9 are considered unlucky, not because 13 can be represented as sum of these two perfect squares but because of their pronunciation. In Japanese, four is shi, which is pronounced the same as the word for death; nine is ku, which sounds the same as the word for torture. And speaking of torture, it was not unusual in times past for bakers to come in for stiff punishment if they shortchanged their customers. In ancient Egypt, someone found selling light loaves might end up with his ear nailed to a doorpost, while in medieval Britain the punishment was likely to be a spell in the pillory. This led to the custom of adding a thirteenth loaf to every batch of 12 to be on the safe side, and hence the expression "a baker's dozen."


Mathematically, the reverse of the square of 13 is the same as the square of the reverse of 13: 132 = 169; the reverse of 169 is 961 and the reverse of 13 is 31; 312 = 961.


Thirteen is the smallest prime number that can be expressed as the sum of the squares of two prime numbers: 13 = 22 + 32. Also the sum of all prime numbers up to 13 (2 + 3 + 5 + 7 + 11 + 13) is equal to the thirteenth prime number (41), and this is the largest such number. On the anagrammatical front is this nice equation: ELEVEN + TWO = TWELVE + ONE.