• 90 can be written using four 4's:

90 has 12 divisors (see below), whose sum is σ = 234. Its totient is φ = 24.

The previous prime is 89. The next prime is 97. The reversal of 90 is 9.

90 = 2^{2} + 3^{2} + ... + 6^{2}.

90 is nontrivially palindromic in base 14.

90 is an esthetic number in base 12, because in such base its adjacent digits differ by 1.

It can be written as a sum of positive squares in only one way, i.e., 81 + 9 = 9^2 + 3^2 .

It is a Harshad number since it is a multiple of its sum of digits (9).

It is a super Niven number, because it is divisible the sum of any subset of its (nonzero) digits.

It is an alternating number because its digits alternate between odd and even.

It is the 16-th Perrin number.

It is a Curzon number.

90 is a nontrivial repdigit in base 14.

It is a plaindrome in base 4, base 7, base 13, base 14 and base 16.

It is a nialpdrome in base 5, base 9, base 10, base 11, base 12, base 14 and base 15.

It is a zygodrome in base 4 and base 14.

It is a polite number, since it can be written in 5 ways as a sum of consecutive naturals, for example, 16 + ... + 20.

It is a pronic number, being equal to 9×10.

It is a practical number, because each smaller number is the sum of distinct divisors of 90, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (117).

90 is an abundant number, since it is smaller than the sum of its proper divisors (144).

It is a pseudoperfect number, because it is the sum of a subset of its proper divisors.

90 is a wasteful number, since it uses less digits than its factorization.

90 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 13 (or 10 counting only the distinct ones).

The product of its (nonzero) digits is 9, while the sum is 9.

The square root of 90 is about 9.4868329805. The cubic root of 90 is about 4.4814047466.

The spelling of 90 in words is "ninety", and thus it is an aban number, an oban number, and an uban number.

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