Professor Cardy

Prime Number Calculator

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Enter an integer (max. 1,000,000)


21391 is a prime number.

About the Calculator

This calculator performs the decomposition of a number into its prime factors and lists all its natural divisors. Enter an integer between 1 and 1,000,000.

Curiosities

1) Relation to multiples of 6: Every prime number greater than 3 is always one less or one more than a multiple of 6.
Example: `21391` is 1 more than `21390` (multiple of 6).

2) Square minus 1 divisible by 24: For every prime number greater than 3, if 1 is subtracted from its square, the result is always divisible by 24.
Example: The square of `21391` is `457574881`. Subtracting 1, we get `457574880`. Note that `457574880 ÷ 24 = 19065620`.

3) Difference of squares: Every prime number greater than 2 can be expressed as the difference of the squares of two consecutive natural numbers.
Example: `21391 = 10696^2 - 10695^2 = 114404416 - 114383025`

4) Form `4n+1` or `4n-1`: Except for the number 2, all prime numbers follow the pattern `4n+1` or `4n-1`, where `n` is a natural number.
Example: `21391` is of the form `4n-1`, where `n = 5348`.

The First 10,000 Prime Numbers