This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
| n | a(n) |
|---|---|
| 0 | 5 |
| 1 | 45 |
| 2 | 405 |
| 3 | 4005 |
| 4 | 40010 |
| 5 | 400005 |
| 6 | 4000005 |
| 7 | 40000010 |
| 8 | 400000015 |
| 9 | 4000000015 |
| 10 | 40000000015 |
| 11 | 400000000015 |
| 12 | 4000000000010 |
| 13 | 40000000000015 |
| 14 | 400000000000020 |
| 15 | -1 |
| 16 | -1 |
| 17 | -1 |
| 18 | 4000000000000000020 |
| 19 | 40000000000000000015 |
| 20 | -1 |