645040021 has 4 divisors (see below), whose sum is σ = 654667552. Its totient is φ = 635412492.

The previous prime is 645040007. The next prime is 645040073. The reversal of 645040021 is 120040546.

645040021 is digitally balanced in base 2, because in such base it contains all the possibile digits an equal number of times.

It is a semiprime because it is the product of two primes, and also a Blum integer, because the two primes are equal to 3 mod 4.

It is a cyclic number.

It is not a de Polignac number, because 645040021 - 2⁹ = 645039509 is a prime.

It is a Duffinian number.

It is a junction number, because it is equal to n+sod(n) for n = 645039974 and 645040001.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (645040001) 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, 4813665 + ... + 4813798.

It is an arithmetic number, because the mean of its divisors is an integer number (163666888).

Almost surely, 2^645040021 is an apocalyptic number.

It is an amenable number.

645040021 is a deficient number, since it is larger than the sum of its proper divisors (9627531).

645040021 is an equidigital number, since it uses as much as digits as its factorization.

645040021 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 9627530.

The product of its (nonzero) digits is 960, while the sum is 22.

The square root of 645040021 is about 25397.6380988469. The cubic root of 645040021 is about 864.0301294970.

Adding to 645040021 its reverse (120040546), we get a palindrome (765080567).

The spelling of 645040021 in words is "six hundred forty-five million, forty thousand, twenty-one".