13 (Thirteen) is the natural number following 12 and preceding 14.
Thirteen is the 6th smallest prime number, the next is seventeen. 13 is a Wilson prime. 13 is the fifth Mersenne prime exponent, yielding 8191. 13 is also the third lucky prime.
13 is a star number and the seventh Fibonacci number. There are 13 Archimedean solids.
13 goes into 999,999 exactly 76,923 times, so fractions with 13 in the denominator have six digit repeating sequences in their decimal expansions.
At 13, the Mertens function sets a new low of -3, subseded later at 31 with a value of -4.
As lucky, unlucky, or significant number
Regarded as an unlucky number in many cultures. Unreasoned fear of the number 13 is termed triskaidekaphobia. Due to this fear, some tall buildings have resorted to skipping the "thirteenth floor", simply by numbering it "14" (though it's really still the thirteenth floor) or by designating the floor "12a" or similar instead. The thirteenth of a month is likewise ominous, particularly when it falls on a Friday (see Friday the 13th). The 13th falls on a Friday more often than any other day. In fact it occurs more often than any other day/number combination except those that coincide with it (Thursday the 12th, Saturday the 14th, etc). On months with a Friday the 13th, the first day of the month begins on a Sunday. This makes calendars that start on Sunday have a full first line, and thus a more square, completed look for that month.
In the Christian religion, the number is considered significant since there were thirteen present at the Last Supper. Also, if both Judas and Matthias are counted, there were thirteen apostles. Thirteen was also once associated with the Epiphany by Christians, the Christ child having received the Magi on his thirteenth day of life.
Modern day Witches have reclaimed the number 13th as a lucky and significant number. This may be similar to Witches reclaiming of other previously negative terms and ideas associated with witchcraft, such as black cats, brooms, and even the term Witch itself. In modern day Wicca, thirteen is considered the maximum size of a coven, and in some traditions the ideal number of members. It is said that, in Gardnerian Wicca, the ideal coven has thirteen members, evenly divided between male and female.
Amongst some Chinese people, it is regarded as a lucky number because it is synonymous with "實生", which means "must be alive". But this belief is not universal, or even known to all Chinese. Also, 13 is regarded as unlucky by those Chinese under the influence of foreign cultures.
In other fields
- The atomic number of aluminium.
- The number of loaves in a "baker's dozen".
- The ASCII and Unicode value for carriage return.
- 'Unlucky for some', according to bingo callers.
- The number of dimensions in some theories of relativity.
- The number of players in a rugby league team.
- The age of maturity in Jewish tradition. Boys get their bar mitzvah on the Shabbat after their thirteenth birthdays.
- The number of steps on the pyramid on the back of the American $1 Bill
- The number of original colonies the United States was founded from. The original flag had thirteen stars, one for each state. New stars have since been added whenever a new state joins the union, but the idea of adding stripes for new states was soon dropped, so the American flag to this day has thirteen horizontal stripes, six white ones and seven red ones.
- The number of guns in a gun salute to U.S. Army, Air Force and Marine Corps Major Generals, and Navy and Coast Guard Rear Admirals Upper Half.
- The number of Plutonium slugs in Fat Man, the atomic bomb dropped on Nagasaki.
- Thirteen, a 2003 movie by Catherine Hardwicke.
- Thirteen Days, a 2000 movie about the Cuban Missile Crisis
- The Thirteenth Floor, a 1999 movie.
- In the title of the movies:
- The 13th moon of Jupiter is Elara.
- Strauss and Howe named Generation X the "Thirteenth Generation".
- The number on a Hell's Angel's patch or tattoo that refers to the thirteenth letter of the alphabet, M, for "marijuana".
- 13, or "X3", is the number of the Mexican street gang Sureņos. It refers to the thirteenth letter of the alphabet, M, for Mexico.
- In rugby union one of the centres, most often but not always the outside centre, wears the 13 shirt.
- Historical years: A.D. 13, 13 B.C., or 1913.