> 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/u/ultra_useful_primes.md).

# Ultra useful primes

```ruby
say(" n   k")
say("----------")
 
for n in (1..13) {
    var t = 2**(2**n)
    printf("%2d   %d\n", n, {|k| t - k -> is_prob_prime }.first)
}
```

#### Output:

```
 n   k
----------
 1   1
 2   3
 3   5
 4   15
 5   5
 6   59
 7   159
 8   189
 9   569
10   105
11   1557
12   2549
13   2439
```

(takes \~20 seconds)