99991021 has 4 divisors (see below), whose sum is σ = 107682652. Its totient is φ = 92299392.

The previous prime is 99991009. The next prime is 99991033. The reversal of 99991021 is 12019999.

It is a semiprime because it is the product of two primes, and also an emirpimes, since its reverse is a distinct semiprime: 12019999 = 587 ⋅20477.

It is an interprime number because it is at equal distance from previous prime (99991009) and next prime (99991033).

It can be written as a sum of positive squares in 2 ways, for example, as 50836900 + 49154121 = 7130^2 + 7011^2 .

It is a cyclic number.

It is not a de Polignac number, because 99991021 - 2⁷ = 99990893 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 (99991081) 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, 3845796 + ... + 3845821.

It is an arithmetic number, because the mean of its divisors is an integer number (26920663).

Almost surely, 2^99991021 is an apocalyptic number.

It is an amenable number.

99991021 is a deficient number, since it is larger than the sum of its proper divisors (7691631).

99991021 is a wasteful number, since it uses less digits than its factorization.

99991021 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 7691630.

The product of its (nonzero) digits is 13122, while the sum is 40.

The square root of 99991021 is about 9999.5510399217. The cubic root of 99991021 is about 464.1449906701.

The spelling of 99991021 in words is "ninety-nine million, nine hundred ninety-one thousand, twenty-one".