20388721 has 4 divisors (see below), whose sum is σ = 20599012. Its totient is φ = 20178432.

The previous prime is 20388583. The next prime is 20388727. The reversal of 20388721 is 12788302.

It is a semiprime because it is the product of two primes, and also an emirpimes, since its reverse is a distinct semiprime: 12788302 = 2 ⋅6394151.

It can be written as a sum of positive squares in 2 ways, for example, as 5948721 + 14440000 = 2439^2 + 3800^2 .

It is a cyclic number.

It is not a de Polignac number, because 20388721 - 2²¹ = 18291569 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (20388727) 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, 105000 + ... + 105193.

It is an arithmetic number, because the mean of its divisors is an integer number (5149753).

Almost surely, 2^20388721 is an apocalyptic number.

It is an amenable number.

20388721 is a deficient number, since it is larger than the sum of its proper divisors (210291).

20388721 is an equidigital number, since it uses as much as digits as its factorization.

20388721 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 210290.

The product of its (nonzero) digits is 5376, while the sum is 31.

The square root of 20388721 is about 4515.3871373339. The cubic root of 20388721 is about 273.1890750111.

The spelling of 20388721 in words is "twenty million, three hundred eighty-eight thousand, seven hundred twenty-one".