20391999 has 4 divisors (see below), whose sum is σ = 27189336. Its totient is φ = 13594664.

The previous prime is 20391979. The next prime is 20392013. The reversal of 20391999 is 99919302.

It is a happy number.

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 20391999 - 2⁵ = 20391967 is a prime.

It is a self number, because there is not a number n which added to its sum of digits gives 20391999.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (20391929) 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, 3398664 + ... + 3398669.

It is an arithmetic number, because the mean of its divisors is an integer number (6797334).

Almost surely, 2^20391999 is an apocalyptic number.

20391999 is a deficient number, since it is larger than the sum of its proper divisors (6797337).

20391999 is an equidigital number, since it uses as much as digits as its factorization.

20391999 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 6797336.

The product of its (nonzero) digits is 39366, while the sum is 42.

The square root of 20391999 is about 4515.7501038034. The cubic root of 20391999 is about 273.2037148995.

The spelling of 20391999 in words is "twenty million, three hundred ninety-one thousand, nine hundred ninety-nine".