Forty
40 is the natural number following 39 and preceding 41. In English, forty is the only number whose constituent letters appear in alphabetical order.
In mathematics
Forty is an octagonal number, and as the sum of the first four pentagonal numbers, it is a pentagonal pyramidal number. It is a Harshad number.
Given 40, the Mertens function returns 0. 40 is the smallest number n with exactly 9 solutions to the equation φ(x) = n.
Forty is the number of n-Queens Problem solutions for n = 7.
In other fields
Forty is:
- the atomic number of zirconium.
- in Judaic, Christian and Islamic tradition: the tag Forty days and forty nights describes the period for which rain fell during Noah's flood, the period Moses spent on Mount Sinai receiving the Ten Commandments and the period Christ spent in the wilderness. In modern Western Christian practice, Lent consists of the 40 days preceding Easter
- in the maxim "Life begins at forty".
- in the expression "Forty winks", meaning a short sleep.
- the number of years of marriage as the ruby wedding anniversary.
- the code for direct dial international phone calls to Romania.
- in the designation of Interstate 40, a freeway that runs from California to North Carolina.
- in the designation for US Highway 40, the 800-mile highway that runs from Baltimore, Maryland, to Vandalia, Illinois, being synonymous with the National Highway.
- the year A.D. 40, 40 BC, or (for example) 1940.
- the title of a song by U2.
- in the name of WD-40, a type of spray lubricant.
- in the name of the food additive FD & C Red Dye #40, commonly known as "Red 40"
- the 40 ounce size used for liquor, typically fairly cheap malt beer
Negative forty is:
- the temperature (in degrees) at which Celsius and Fahrenheit scales are equivalent (often referred to as minus forty or 40 below).
