> 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/m/modular_exponentiation.md).

# Modular exponentiation

Built-in:

```ruby
say expmod(
    2988348162058574136915891421498819466320163312926952423791023078876139,
    2351399303373464486466122544523690094744975233415544072992656881240319,
    10**40)
```

User-defined:

```ruby
func expmod(a, b, n) {
    var c = 1
    do {
        (c *= a) %= n if b.is_odd
        (a *= a) %= n
    } while (b //= 2)
    c
}
```

#### Output:

```
1527229998585248450016808958343740453059
```