25 (number)

25 (twenty-five) is the natural number following 24 and preceding 26.

In mathematics

It is a square number, being 5² = 5 × 5, and it is also the sum of two squares, 3² + 4², often used in demonstrations of the Pythagorean theorem. It is the smallest square that is also a sum of two squares. 25 is a centered octagonal number and an automorphic number.

It is the smallest base 10 Friedman number as it can be expressed by its own numbers: 5².

It is also a Cullen number. 25 is the smallest pseudoprime satisfying the congruence 7ⁿ = 7 mod n. It is also a Somer pseudoprime based on 6.

25 is an aspiring number - its aliquot sequence converts to a perfect number. It is the smallest aspiring number.

Within base 10 one can readily test for divisibility by 25 by seeing if the last two digits of the number match 25, 50, 75 or 00.

In science

• The atomic number of manganese

Astronomy

• Messier object M25, a magnitude 4.9 open cluster in the constellation Sagittarius

• The New General Catalogue object NGC 25, a magnitude 11.9 barred spiral galaxy in the constellation Phoenix

• The Saros number of the solar eclipse series which began on -2051 April 19 and ended on -771 May 26. The duration of Saros series 25 was 1280.1 years, and it contained 72 solar eclipses.

• The Saros number of the lunar eclipse series which began on -2038 August 6 and ended on -469 March 5. The duration of Saros series 25 was 1568.6 years, and it contained 88 lunar eclipses.

In other fields

Twenty-five is:

• The number of years of marriage marked in a silver wedding anniversary.
• The minimum age of candidates for election to the United States House of Representatives. "Under 25" provides a common cut-off point for designating youth.
• The number of cents in a quarter, an American coin with George Washington's portrait on the obverse and an eagle on the reverse (or special reverses for the special state quarters since 1999), and a Canadian coin with Queen Elizabeth II on the obverse and a caribou on the reverse (or special reverses lately).
• The usual TCP port for SMTP.
• The designation of United States Interstate 25, a freeway that runs from New Mexico to Wyoming.
• The designation of the M25 London Orbital motorway.
• The (critical) number of Florida electoral votes for the 2000 U.S. presidential election.
• The name of the national card game of Ireland. Twenty-five is played widely in rural pubs and at home. The game moves quite quickly - it does not take long to play a hand - and although there is a fair amount of luck in it, there is also some scope for skill. It is played by four people. The ranking order of the cards may seem a little arcane at first, but with a little practice it becomes second nature. Apart from the scoring it is more or less the same as the old games of Maw and Spoil Five.
• The size of the full roster on a major league baseball team
• The number of playable characters in Super Smash Bros. Melee, if Zelda and Shiek count as one character.
• In Pokémon, the Pokédex I.D. number of Pikachu.
• The speed limit in Nutbush, Tennessee, according to Tina Turner's song Nutbush City Limits.
• "25", a song by Veruca Salt from their 1994 album American Thighs
• In Sweden, the minimum age to be sterilized
• The new raid size in World of Warcraft expansion, World of Warcraft: The Burning Crusade.
• TwentyFive is the name of the new George Michael compilation celebrating 25 years in the music business (1981-2006)

Historical years

25 A.D., 25 B.C., 1925, 2025, etc.

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