21037991 has 4 divisors (see below), whose sum is σ = 22656312. Its totient is φ = 19419672.

The previous prime is 21037967. The next prime is 21038009. The reversal of 21037991 is 19973012.

It is a happy number.

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

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21037991 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 (21037901) 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, 809141 + ... + 809166.

It is an arithmetic number, because the mean of its divisors is an integer number (5664078).

Almost surely, 2^21037991 is an apocalyptic number.

21037991 is a deficient number, since it is larger than the sum of its proper divisors (1618321).

21037991 is a wasteful number, since it uses less digits than its factorization.

21037991 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 1618320.

The product of its (nonzero) digits is 3402, while the sum is 32.

The square root of 21037991 is about 4586.7189798373. The cubic root of 21037991 is about 276.0586892976.

The spelling of 21037991 in words is "twenty-one million, thirty-seven thousand, nine hundred ninety-one".