Professor Cardy

Prime Number Calculator

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


61981 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: `61981` is 1 more than `61980` (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 `61981` is `3841644361`. Subtracting 1, we get `3841644360`. Note that `3841644360 ÷ 24 = 160068515`.

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: `61981 = 30991^2 - 30990^2 = 960442081 - 960380100`

4) Sum of two squares: Every prime number of the form `4n+1` (where n is a positive natural number) can be expressed as the sum of two squares.
Example: `61981 = 85^2 + 234^2 = 7225 + 54756`, where `61981 = 4 × 15495 + 1`.

5) 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: `61981` is of the form `4n+1`, where `n = 15495`.

The First 10,000 Prime Numbers