• 645 can be written using four 4's:

645 has 8 divisors (see below), whose sum is σ = 1056. Its totient is φ = 336.

The previous prime is 643. The next prime is 647. The reversal of 645 is 546.

645 = T_{6} + T_{7} + ... +
T_{15}.

645 = 3^{2} + 4^{2} + ... + 12^{2}.

645 is nontrivially palindromic in base 2.

It is an interprime number because it is at equal distance from previous prime (643) and next prime (647).

It is a sphenic number, since it is the product of 3 distinct primes.

It is not a de Polignac number, because 645 - 2^{1} = 643 is a prime.

It is a Smith number, since the sum of its digits (15) coincides with the sum of the digits of its prime factors. Since it is squarefree, it is also a hoax number.

It is a Harshad number since it is a multiple of its sum of digits (15), and also a Moran number because the ratio is a prime number: 43 = 645 / (6 + 4 + 5).

It is a Curzon number.

645 is a lucky number.

It is a plaindrome in base 12.

It is a congruent number.

It is an inconsummate number, since it does not exist a number *n* which divided by its sum of digits gives 645.

It is not an unprimeable number, because it can be changed into a prime (641) by changing a digit.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 7 + ... + 36.

It is an arithmetic number, because the mean of its divisors is an integer number (132).

It is a Poulet number, since it divides 2^{644}-1.

645 is the 15-th octagonal number.

It is an amenable number.

645 is a deficient number, since it is larger than the sum of its proper divisors (411).

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

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

The sum of its prime factors is 51.

The product of its digits is 120, while the sum is 15.

The square root of 645 is about 25.3968501984. The cubic root of 645 is about 8.6401225977.

Subtracting from 645 its sum of digits (15), we obtain a triangular number (630 = T_{35}).

Subtracting from 645 its product of digits (120), we obtain a palindrome (525).

Subtracting from 645 its reverse (546), we obtain a palindrome (99).

The spelling of 645 in words is "six hundred forty-five", and thus it is an aban number.

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