645551 has 4 divisors (see below), whose sum is σ = 648600. Its totient is φ = 642504.

The previous prime is 645529. The next prime is 645571. The reversal of 645551 is 155546.

It is a semiprime because it is the product of two primes, and also an emirpimes, since its reverse is a distinct semiprime: 155546 = 2 ⋅77773.

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-645551 is a prime.

It is a Duffinian number.

It is a self number, because there is not a number n which added to its sum of digits gives 645551.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (645521) 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, 1181 + ... + 1638.

It is an arithmetic number, because the mean of its divisors is an integer number (162150).

2⁶⁴⁵⁵⁵¹ is an apocalyptic number.

645551 is a deficient number, since it is larger than the sum of its proper divisors (3049).

645551 is a wasteful number, since it uses less digits than its factorization.

645551 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 3048.

The product of its digits is 3000, while the sum is 26.

The square root of 645551 is about 803.4618845969. The cubic root of 645551 is about 86.4258221144.

The spelling of 645551 in words is "six hundred forty-five thousand, five hundred fifty-one".