15 (number)

15 (fifteen) is the natural number following 14 and preceding 16. In English, it is the smallest natural number with seven letters in its spelled out name.

In mathematics

Fifteen is a triangular number and a hexagonal number, a pentatope number, and the 4th Bell number. Fifteen is the double factorial of 5. It is a composite number; its proper divisors being 1|, 3 and 5. With only two exceptions, all prime quadruplets enclose a multiple of 15, with 15 itself being enclosed by the quadruplet (11, 13, 17, 19). 15 is also the number of supersingular primes.

15 is the 4th discrete biprime (3.5) and the first member of the (3.q) discrete biprime family. The number proceeding 15; 14 is itself a discrete biprime and this is the first such pair of discrete biprimes. The next example is the pair commencing 21.

The aliquot sum of 15 is 9, a square prime 15 has an aliquot sequence of 6 members (15,9,4,3,1,0). 15 is the fourth composite number in the 3-aliquot tree. The abundant 12 is also a member of this tree. Fifteen is the aliquot sum of the consecutive 4-power 16, and the discrete biprime 33.

15 and 16 form a Ruth-Aaron pair under the second definition in which repeated prime factors are counted as often as they occur.

Fifteen is the magic constant of the unique order-3 normal magic square:

15 is the smallest number that can be factorized using Shor's quantum algorithm.

There are 15 solutions to Znám's problem of length 7.

15 is a repdigit in binary (1111). In hexadecimal, as well as all higher bases, fifteen is represented as F.

In science

• The atomic number of phosphorus.
• Group 15 of the periodic table are sometimes known as the "Pnictogens".

In religion

• There were 15 steps between the Ezrat Nashim and the Courtyard of the Temple in Jerusalem, on which the Levites stood and sang during the offering of sacrifices. Fifteen Psalms (Chapters 120-134), collectively known as the "Songs of Ascent", correspond to the 15 steps on which the Levites stood.
• In the Hebrew numbering system, the number 15 is not written according to the usual method, with the letters that represent "10" and "5" (י-ה, yodh and heh), because those spell out one of the Jewish names of God. Instead, the date is written with the letters representing "9" and "6" (ט-ו, teth and vav)
• Passover begins on the 15th day of the Hebrew month of Nisan
• Sukkot begins on the 15th day of the Hebrew month of Tishrei
• Tu Bishvat is a Jewish holiday occurring on the 15th of the Hebrew month of Shevat
• Shushan Purim (the day on which Purim is celebrated in Jerusalem and a few other cities in Israel) occurs on the 15th day of the Hebrew month of Adar
• Tu B'Av is a minor Jewish holiday occurring on the 15th day of the Hebrew month of Av

In Sports

• In tennis, the number 15 represents the first point gained in a game.
• In Rugby Union the full back wears the number 15, it is also the number of players on the field.
• Baseball player Frank Duffy, shortstop, wore uniform number 15 for the Cincinnati Reds (1970-71) and Cleveland Indians (1972-77).
• Basketball player Carmelo Anthony wears number 15 for the Denver Nuggets and for Team USA Basketball.
• Baseball center fielder Jim Edmonds wears the number 15
• Basketball player Vince Carter wears number 15 for the New Jersey Nets.
• Football player Tim Tebow wears number 15 for the college team, the Florida Gators
• Football player Colt Brennan wore number 15 for the college team, the Hawaii Warriors
• Football player Bart Starr wore number 15 as Quarterback for the Green Bay Packers

The age 15

Age 15 is:

• The age of a quinceañera, a Hispanic girl celebrating her fifteenth birthday. Thus, Spanish bingo callers might refer to the number 15 as la niña bonita (the beautiful girl).
• The age for obtaining a driver's (or learner's) permit in certain jurisdictions, including some where the age for a driver's license is sixteen.
• In the UK a minor can be sent to prison to await trial at the age of 15
• In some cultures 15 is when childhood officially ends, from this age onwards (till 18) they are called young people

In other fields

Fifteen is:

• The number 15 is a major theme for the Marilyn Manson album Mechanical Animals, which refers to the number numerous times.
• The number of days in each of the 24 cycles of the Chinese calendar.
• The number of guns in a gun salute to Army, Marine Corps, and Air Force Lieutenant Generals, and Navy and Coast Guard Vice Admirals.
• The designation of Interstate 15, a freeway that runs from California to Montana.
• The '15 refers to the first Jacobite Rising in Great Britain in 1715.
• The number of checkers each side has in the beginning of a backgammon game.
• The 15 puzzle.
• Fifteen, a television program.
• A restaurant in London, built by celebrity chef Jamie Oliver
• In the United States, the customary percentage to tip a waitress in a restaurant by one or two guests
• 15 minutes of fame
• The 15th is a Wire song
• The number of balls in the eight ball variant of billiards
• The number of function keys on most Mac keyboards
• The name of a modern punk band, Fifteen (band), with lead singer Jeff Ott.
• The number of letters in the words "uncopyrightable", "dermatoglyphics", "misconjugatedly", and "hydropneumatics", which are the longest words there are without repeating a letter.
• 15 is one of The Numbers - 4, 8, 15, 16, 23, and 42 - featured in Lost.
• The number of completed, numbered symphonies and string quartets by Dmitri Shostakovich
• A film rating and age limit on viewing given by the British Board of Film Classification
• The number to dial for SAMU in case of an emergency in France.
• The number of minutes in one quarter of an hour; 15 minutes past or before an hour is often known as quarter past and quarter to, respectively.
• Fifteen men on the dead man's chest is the first verse of the chorus of a fictional sea shanty from Robert Louis Stevenson's novel Treasure Island (1883),
• 15 Step is the opening song on the Radiohead album, In Rainbows.
• In casual conversation, an arbitrary number used to refer to a large quantity of things. Often prefixed with "like". Greg Oden - 15 titles, Katharine McPhee - 15 babies, 15 Terminators
• The number of the French department Cantal

Historical years

15 A.D., 15 B.C., 1915, 2015, etc.

References

• Wells, D. The Penguin Dictionary of Curious and Interesting Numbers London: Penguin Group. (1987): 91 - 93