• 99 can be written using four 4's:

99 has 6 divisors (see below), whose sum is σ = 156. Its totient is φ = 60.

The previous prime is 97. The next prime is 101.

99 is nontrivially palindromic in base 2 and base 10.

99 is digitally balanced in base 4, because in such base it contains all the possibile digits an equal number of times.

It is a Cunningham number, because it is equal to 10^{2}-1.

99 is an esthetic number in base 13, because in such base its adjacent digits differ by 1.

It is an interprime number because it is at equal distance from previous prime (97) and next prime (101).

It is a trimorphic number since its cube, 970299, ends in 99.

It is not a de Polignac number, because 99 - 2^{1} = 97 is a prime.

It is a nude number because it is divisible by every one of its digits.

It is an Ulam number.

99 is strictly pandigital in base 4.

99 is a lucky number.

99 is a nontrivial repdigit in base 10.

It is a plaindrome in base 5, base 10, base 13 and base 15.

It is a nialpdrome in base 10, base 11, base 12, base 14 and base 16.

It is a zygodrome in base 2 and base 10.

It is a Kaprekar number, because its square (9801) can be partitioned into two parts whose sum is 99.

It is a polite number, since it can be written in 5 ways as a sum of consecutive naturals, for example, 4 + ... + 14.

It is an arithmetic number, because the mean of its divisors is an integer number (26).

99 is a deficient number, since it is larger than the sum of its proper divisors (57).

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

With its predecessor (98) it forms an eRAP, since the sums of their prime factors are consecutive (16 and 17).

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

The sum of its prime factors is 17 (or 14 counting only the distinct ones).

The product of its digits is 81, while the sum is 18.

The square root of 99 is about 9.9498743711. The cubic root of 99 is about 4.6260650092.

Subtracting from 99 its sum of digits (18), we obtain a 4-th power (81 = 3^{4}).

The spelling of 99 in words is "ninety-nine", and thus it is an aban number, an oban number, and an uban number.

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