645721 has 4 divisors (see below), whose sum is σ = 647332. Its totient is φ = 644112.

The previous prime is 645713. The next prime is 645727. The reversal of 645721 is 127546.

It is a semiprime because it is the product of two primes, and also a brilliant number, because the two primes have the same length, and also an emirpimes, since its reverse is a distinct semiprime: 127546 = 2 ⋅63773.

It can be written as a sum of positive squares in 2 ways, for example, as 190096 + 455625 = 436^2 + 675^2 .

It is a cyclic number.

It is not a de Polignac number, because 645721 - 2³ = 645713 is a prime.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (645727) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 331 + ... + 1183.

It is an arithmetic number, because the mean of its divisors is an integer number (161833).

2⁶⁴⁵⁷²¹ is an apocalyptic number.

It is an amenable number.

645721 is a deficient number, since it is larger than the sum of its proper divisors (1611).

645721 is an equidigital number, since it uses as much as digits as its factorization.

645721 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 1610.

The product of its digits is 1680, while the sum is 25.

The square root of 645721 is about 803.5676698325. The cubic root of 645721 is about 86.4334079340.

The spelling of 645721 in words is "six hundred forty-five thousand, seven hundred twenty-one".