> 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/n/nice_primes.md).

# Nice primes

```ruby
func digital_root(n, base=10) {
    while (n.len(base) > 1) {
        n = n.sumdigits(base)
    }
    return n
}

say primes(500, 1000).grep { digital_root(_).is_prime }
```

## Output:

```
[509, 547, 563, 569, 587, 599, 601, 617, 619, 641, 653, 659, 673, 677, 691, 709, 727, 743, 761, 797, 821, 839, 853, 857, 887, 907, 911, 929, 941, 947, 977, 983, 997]
```