20400021 has 24 divisors (see below), whose sum is σ = 31150080. Its totient is φ = 12843936.

The previous prime is 20400019. The next prime is 20400031. The reversal of 20400021 is 12000402.

It is not a de Polignac number, because 20400021 - 2¹ = 20400019 is a prime.

It is a Harshad number since it is a multiple of its sum of digits (9).

It is a junction number, because it is equal to n+sod(n) for n = 20399976 and 20400012.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (20400031) by changing a digit.

It is a polite number, since it can be written in 23 ways as a sum of consecutive naturals, for example, 11436 + ... + 13098.

It is an arithmetic number, because the mean of its divisors is an integer number (1297920).

Almost surely, 2^20400021 is an apocalyptic number.

It is an amenable number.

20400021 is a deficient number, since it is larger than the sum of its proper divisors (10750059).

20400021 is a wasteful number, since it uses less digits than its factorization.

20400021 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 1745 (or 1742 counting only the distinct ones).

The product of its (nonzero) digits is 16, while the sum is 9.

The square root of 20400021 is about 4516.6382409930. The cubic root of 20400021 is about 273.2395353680.

Adding to 20400021 its reverse (12000402), we get a palindrome (32400423).

The spelling of 20400021 in words is "twenty million, four hundred thousand, twenty-one".