Is 645 a prime number? What are the divisors of 645?

## Parity of 645

645 is an odd number, because it is not evenly divisible by 2.

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## Is 645 a perfect square number?

A number is a perfect square (or a square number) if its square root is an integer; that is to say, it is the product of an integer with itself. Here, the square root of 645 is about 25.397.

Thus, the square root of 645 is not an integer, and therefore 645 is not a square number.

## What is the square number of 645?

The square of a number (here 645) is the result of the product of this number (645) by itself (i.e., 645 × 645); the square of 645 is sometimes called "raising 645 to the power 2", or "645 squared".

The square of 645 is 416 025 because 645 × 645 = 6452 = 416 025.

As a consequence, 645 is the square root of 416 025.

## Number of digits of 645

645 is a number with 3 digits.

## What are the multiples of 645?

The multiples of 645 are all integers evenly divisible by 645, that is all numbers such that the remainder of the division by 645 is zero. There are infinitely many multiples of 645. The smallest multiples of 645 are:

## How to determine whether an integer is a prime number?

To determine the primality of a number, several algorithms can be used. The most naive technique is to test all divisors strictly smaller to the number of which we want to determine the primality (here 645). First, we can eliminate all even numbers greater than 2 (and hence 4, 6, 8…). Then, we can stop this check when we reach the square root of the number of which we want to determine the primality (here the square root is about 25.397). Historically, the sieve of Eratosthenes (dating from the Greek mathematics) implements this technique in a relatively efficient manner.

More modern techniques include the sieve of Atkin, probabilistic algorithms, and the cyclotomic AKS test.

## Numbers near 645

• Preceding numbers: …643, 644
• Following numbers: 646, 647

### Nearest numbers from 645

• Preceding prime number: 643
• Following prime number: 647
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