> For the complete documentation index, see [llms.txt](https://trizen.gitbook.io/sidef-lang/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://trizen.gitbook.io/sidef-lang/programming_tasks/p/primality_by_wilsons_theorem.md).

# Primality by Wilson's theorem

```ruby
func is_wilson_prime_slow(n) {
    n > 1 || return false
    (n-1)! % n == n-1
}

func is_wilson_prime_fast(n) {
    n > 1 || return false
    factorialmod(n-1, n) == n-1
}

say 25.by(is_wilson_prime_slow)     #=> [2, 3, 5, ..., 83, 89, 97]
say 25.by(is_wilson_prime_fast)     #=> [2, 3, 5, ..., 83, 89, 97]

say is_wilson_prime_fast(2**43 - 1)   #=> false
say is_wilson_prime_fast(2**61 - 1)   #=> true
```