90

Math

is the integer between.

Math features

 * is an even number.
 * 90 is an abundant number and a composite number.
 * 90 is a palindromic number in Roman numerals, base 14, 17, 29, 44 and base 89.
 * 90 is a Harshad number.
 * 90 has 3 distinct prime factors, but not squarefree, so it is not a sphenic number,
 * 90 is a pronic number, equal to \(9\times10\).
 * 90 is a unitary perfect number, as its unitary divisors (1 + 2 + 5 + 9 + 10 + 18 + 45 = 90) sum up to 90.
 * 90 is a highly abundant number and a regular number, being a factor of 602, which is 3600.
 * 90 is the last multiple of 9 to be Harshad before the first non-Harshad multiple of 9 (which is 99). This also means 90 is the last multiple of 9 whose digit sum is 9 before the first multiple of 9 whose digit sum is 18, and the number, again, is 99.

90 in degrees
In degrees, 90&deg; is the angle for a square and is \(\frac14\) of a full 360&deg;.

&frac12;

 * $1⁄2$ is half of.