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13 (number)

Natural number

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  • 7 chapters

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Through Natural number, Prime number and Twin prime

Folklore surrounding the number 13 appears in many cultures around the world: one theory is that this is due to the cultures employing lunar-solar calendars (there are approximately 12.41 lunations per solar year, and hence 12 "true months" plus a smaller, and often portentous, thirteenth month). This can be witnessed, for example, in the "Twelve Days of Christmas" of Western European tradition.

In mathematics

The number 13 is a prime number, happy number and a lucky number. It is a twin prime with 11, as well as a cousin prime with 17. It is the second of the only three known Wilson primes: 5, 13, and 563. A 13-sided regular polygon is called a tridecagon.

13 is a Fibonacci number and an ordered Bell number. In Sanskrit prosody, there are 13 combinations of light and heavy syllables that add to six morae, since 13 is a Fibonacci number. Because 13 is an ordered bell number, there are 13 orders in which a three-way horse race can finish, allowing for ties.

List of basic calculations

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