2157991 has 4 divisors (see below), whose sum is σ = 2354184. Its totient is φ = 1961800.

The previous prime is 2157989. The next prime is 2158019. The reversal of 2157991 is 1997512.

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 2157991 - 2¹ = 2157989 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 (2157901) 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, 98080 + ... + 98101.

It is an arithmetic number, because the mean of its divisors is an integer number (588546).

2^2157991 is an apocalyptic number.

2157991 is a deficient number, since it is larger than the sum of its proper divisors (196193).

2157991 is a wasteful number, since it uses less digits than its factorization.

2157991 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 196192.

The product of its digits is 5670, while the sum is 34.

The square root of 2157991 is about 1469.0102109924. The cubic root of 2157991 is about 129.2259924956.

The spelling of 2157991 in words is "two million, one hundred fifty-seven thousand, nine hundred ninety-one".