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40 (number)

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40 is the natural number following 39 and preceding 41. In English, forty is the only number whose constituent letters appear in alphabetical order.

Template:Numbers 40s
Cardinal forty
Ordinal40th (fortieth)
Numeral system
Factorization
Divisors 2, 4, 5, 8, 10, 20
Roman numeralXL
Binary101000
Hexadecimal28

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:

Negative forty is:

  • the temperature (in degrees) at which both the Celsius and Fahrenheit scales are at the same numerical point (ie. -40C = -40F Often referred to as minus forty or 40 below).