99990991 has 4 divisors (see below), whose sum is σ = 105872832. Its totient is φ = 94109152.

The previous prime is 99990973. The next prime is 99991009. The reversal of 99990991 is 19909999.

It is a happy number.

It is a semiprime because it is the product of two primes, and also an emirpimes, since its reverse is a distinct semiprime: 19909999 = 47 ⋅423617.

It is an interprime number because it is at equal distance from previous prime (99990973) and next prime (99991009).

It is a cyclic number.

It is not a de Polignac number, because 99990991 - 2²¹ = 97893839 is a prime.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (99990911) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 2940895 + ... + 2940928.

It is an arithmetic number, because the mean of its divisors is an integer number (26468208).

Almost surely, 2^99990991 is an apocalyptic number.

99990991 is a deficient number, since it is larger than the sum of its proper divisors (5881841).

99990991 is a wasteful number, since it uses less digits than its factorization.

99990991 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 5881840.

The product of its (nonzero) digits is 531441, while the sum is 55.

The square root of 99990991 is about 9999.5495398543. The cubic root of 99990991 is about 464.1449442514.

It can be divided in two parts, 999909 and 91, that added together give a 6-th power (1000000 = 10⁶).

The spelling of 99990991 in words is "ninety-nine million, nine hundred ninety thousand, nine hundred ninety-one".