645721998 has 32 divisors (see below), whose sum is σ = 1305991680. Its totient is φ = 212829120.

The previous prime is 645721991. The next prime is 645722017. The reversal of 645721998 is 899127546.

It is a super-2 number, since 2×645721998² = 833913797402224008, which contains 22 as substring.

It is a self number, because there is not a number n which added to its sum of digits gives 645721998.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (645721991) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (19) of ones.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 224586 + ... + 227442.

It is an arithmetic number, because the mean of its divisors is an integer number (40812240).

Almost surely, 2^645721998 is an apocalyptic number.

645721998 is an abundant number, since it is smaller than the sum of its proper divisors (660269682).

It is a pseudoperfect number, because it is the sum of a subset of its proper divisors.

645721998 is a wasteful number, since it uses less digits than its factorization.

645721998 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 3272.

The product of its digits is 1088640, while the sum is 51.

The square root of 645721998 is about 25411.0605445739. The cubic root of 645721998 is about 864.3345246326.

The spelling of 645721998 in words is "six hundred forty-five million, seven hundred twenty-one thousand, nine hundred ninety-eight".