45 (number)

Template:Infobox number 45 (forty-five) is the natural number following 44 and preceding 46.

In mathematics

File:45-square-difference.png

45 as the difference of two nonzero squares (in orange)

45 is a composite number and a Størmer number.

Forty-five is conjectured from Ramsey number $R(5,5)$.

$\varphi (45)=\varphi (\sigma (45))$

Forty-five degrees is half of a right angle (90°).

In music

File:45rpm.jpg

45 rpm gramophone record

• A type of gramophone record classified by its rotational speed of 45 revolutions per minute (rpm)

In other fields

• 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.
• In years of marriage, the sapphire wedding anniversary.
• 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.