|Divisors||1, 3, 5, 9, 15, 45|
Forty-five is a triangular number, and in particular the sum of all the decimal digits (0 + 1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 = 45). It is the smallest triangle number (after 1) which can be written as the sum of two squares. It is also a hexagonal and 16-gonal number,
Since the greatest prime factor of 452 + 1 = 2026 is 1013, which is much more than 45 twice, 45 is a Størmer number.
- Messier object M45, a magnitude 1.4 open cluster in the constellation Taurus, also known as the Pleiades
- The New General Catalogue object NGC 45, a magnitude 10.6 spiral galaxy in the constellation Cetus
- The Saros number of the solar eclipse series which began on −1436 March 30 and ended on −156 May 7. The duration of Saros series 45 was 1280.1 years, and it contained 72 solar eclipses.
- The Saros number of the lunar eclipse series which began on −1369 August 19 and ended on 182 March. The duration of Saros series 45 was 1550.6 years, and it contained 87 lunar eclipses.
- A type of gramophone record classified by its rotational speed of 45 revolutions per minute (rpm)
- The group Stars on 45 and its 1981 Stars on 45 (song)
- Included in the title of "45 and Fat", a 1996 song by Babybird
- The title of a 2000 song by The Atomic Bitchwax, "Forty-Five"
- The title of a 2002 song by Elvis Costello, "45", both referring to the 45 rpm singles and to the artist's age when he wrote the song, which was released when he was 47
- The title of a 2003 song by Shinedown, "45" see 45 (Shinedown song)
- The title of a 2006 song by noodles, "45"
- The title of a 2007 song by Ryan Shaw, "Do the 45"
- The title of a 2007 song by The Saturday Knights, "45"
- The title of a 1982 album by Kino, 45
- The title of a 2012 song by The Gaslight Anthem, "45"
- Repeated continuously in the 1997 song "Brimful of Asha" by Cornershop
In other fields
Forty-five may also refer to:
- The '45 refers to the Jacobite rising of 1745 in Great Britain], or the year that World War II ended, which was 1945.
- A card game: Forty-five
- .45 (film), a 2006 motion picture.
- +45 is the telephone dialing code for Denmark
- 45 (book), a book of essays by record producer Bill Drummond, derived both from the speed of a pop single and from his age when he finished writing it
- A football match consists of two periods of 45 minutes each.
- Guns or ammunition of .45 caliber. In the United States, "45" is often a reference to one of two specific .45 caliber cartridges— the .45 Colt or the .45 ACP.
- I-45 is the designation for a US interstate highway in Texas, connecting the major cities of Dallas and Houston; it is also the shortest "primary" interstate highway (one ending in 0 or 5)
- The number of the French department Loiret
- The maximum mark an International Baccalaureate student can obtain.
- In years of marriage, the sapphire wedding anniversary.
- Forty Five (audio drama) a Big Finish 2008 audio play made for the forty fifth anniversary of the British science fiction television show Doctor Who.
- Issue 45 of The North Briton was thought to be seditious but its publisher, John Wilkes, was celebrated as a champion of liberty. The number 45 was used as a symbol of support for him. Banquets were held with a theme of 45 while many items were produced showing the number or featuring it in some way. For example, a wig was produced with 45 curls.