21157991 has 4 divisors (see below), whose sum is σ = 21184584. Its totient is φ = 21131400.

The previous prime is 21157987. The next prime is 21157993. The reversal of 21157991 is 19975112.

It is a semiprime because it is the product of two primes.

It is a cyclic number.

It is not a de Polignac number, because 21157991 - 2² = 21157987 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 (21157993) 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, 12065 + ... + 13706.

It is an arithmetic number, because the mean of its divisors is an integer number (5296146).

Almost surely, 2^21157991 is an apocalyptic number.

21157991 is a deficient number, since it is larger than the sum of its proper divisors (26593).

21157991 is an equidigital number, since it uses as much as digits as its factorization.

21157991 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 26592.

The product of its digits is 5670, while the sum is 35.

The square root of 21157991 is about 4599.7816252514. The cubic root of 21157991 is about 276.5825710119.

The spelling of 21157991 in words is "twenty-one million, one hundred fifty-seven thousand, nine hundred ninety-one".