645606556921 has 2 divisors, whose sum is σ = 645606556922. Its totient is φ = 645606556920.

The previous prime is 645606556919. The next prime is 645606556961. The reversal of 645606556921 is 129655606546.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 644064031296 + 1542525625 = 802536^2 + 39275^2 .

It is a cyclic number.

It is not a de Polignac number, because 645606556921 - 2¹ = 645606556919 is a prime.

Together with 645606556919, it forms a pair of twin primes.

It is not a weakly prime, because it can be changed into another prime (645606556961) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 322803278460 + 322803278461.

It is an arithmetic number, because the mean of its divisors is an integer number (322803278461).

Almost surely, 2^645606556921 is an apocalyptic number.

It is an amenable number.

645606556921 is a deficient number, since it is larger than the sum of its proper divisors (1).

645606556921 is an equidigital number, since it uses as much as digits as its factorization.

645606556921 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 11664000, while the sum is 55.

The spelling of 645606556921 in words is "six hundred forty-five billion, six hundred six million, five hundred fifty-six thousand, nine hundred twenty-one".