20391721 has 16 divisors (see below), whose sum is σ = 24796800. Its totient is φ = 16369920.

The previous prime is 20391713. The next prime is 20391737. The reversal of 20391721 is 12719302.

It is not a de Polignac number, because 20391721 - 2³ = 20391713 is a prime.

It is not an unprimeable number, because it can be changed into a prime (20391781) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 41286 + ... + 41776.

It is an arithmetic number, because the mean of its divisors is an integer number (1549800).

Almost surely, 2^20391721 is an apocalyptic number.

It is an amenable number.

20391721 is a deficient number, since it is larger than the sum of its proper divisors (4405079).

20391721 is a wasteful number, since it uses less digits than its factorization.

20391721 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 864.

The product of its (nonzero) digits is 756, while the sum is 25.

The square root of 20391721 is about 4515.7193225443. The cubic root of 20391721 is about 273.2024733836.

It can be divided in two parts, 20391 and 721, that added together give a palindrome (21112).

The spelling of 20391721 in words is "twenty million, three hundred ninety-one thousand, seven hundred twenty-one".