## Friday, October 31, 2008

### 13

13 is a prime number.

13 has a representation as a sum of two squares: 13 = 22 + 32.

13 is the 7th Fibonacci number.

11 and 13 form a twin prime pair.

13
is a lucky number. To build the lucky number sequence, start with natural numbers. Delete every second number, leaving 1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, . . . . The second number remaining is 3, so delete every third number, leaving 1, 3, 7, 9, 13, 15, 19, 21, . . . .The next number remaining is 7, so delete every 7th number, leaving 1, 3, 7, 9, 13, 15, 21, . . . . The next number remaining is 9, so delete every ninth number, and so on. The numbers that remain are lucky: 3, 7, 9, 13, 15, 21, 25, . . . .

132 = 169 and (reversing all digits) 312 = 961.

Three planes can cut a doughnut into a maximum of
13 parts.

13 is 111 in base 3.

The fear of the number
13 is called triskaidekaphobia.

There are
13 tricks in each hand of bridge.

Source:
Number Gossip; Prime Curios!

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