645991 has 4 divisors (see below), whose sum is σ = 657000. Its totient is φ = 634984.

The previous prime is 645979. The next prime is 646003. The reversal of 645991 is 199546.

It is a semiprime because it is the product of two primes.

It is an interprime number because it is at equal distance from previous prime (645979) and next prime (646003).

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-645991 is a prime.

It is a Duffinian number.

645991 is a lucky number.

It is a nialpdrome in base 15.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (645901) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 5416 + ... + 5533.

It is an arithmetic number, because the mean of its divisors is an integer number (164250).

2⁶⁴⁵⁹⁹¹ is an apocalyptic number.

645991 is a deficient number, since it is larger than the sum of its proper divisors (11009).

645991 is a wasteful number, since it uses less digits than its factorization.

645991 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 11008.

The product of its digits is 9720, while the sum is 34.

The square root of 645991 is about 803.7356530601. The cubic root of 645991 is about 86.4454532643.

The spelling of 645991 in words is "six hundred forty-five thousand, nine hundred ninety-one".