21393991 has 4 divisors (see below), whose sum is σ = 21408912. Its totient is φ = 21379072.

The previous prime is 21393989. The next prime is 21394063. The reversal of 21393991 is 19939312.

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 21393991 - 2¹ = 21393989 is a prime.

It is a Duffinian number.

It is a self number, because there is not a number n which added to its sum of digits gives 21393991.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21393961) 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, 5050 + ... + 8263.

It is an arithmetic number, because the mean of its divisors is an integer number (5352228).

Almost surely, 2^21393991 is an apocalyptic number.

21393991 is a deficient number, since it is larger than the sum of its proper divisors (14921).

21393991 is a wasteful number, since it uses less digits than its factorization.

21393991 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 14920.

The product of its digits is 13122, while the sum is 37.

The square root of 21393991 is about 4625.3638775776. The cubic root of 21393991 is about 277.6071214818.

The spelling of 21393991 in words is "twenty-one million, three hundred ninety-three thousand, nine hundred ninety-one".