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) |
|---|---|
| 1 | 21 |
| 2 | 291 |
| 3 | 2991 |
| 4 | 29991 |
| 5 | 299991 |
| 6 | 2999991 |
| 7 | 29999991 |
| 8 | 299999991 |
| 9 | 2999999991 |
| 10 | 29999999991 |
| 11 | 299999999991 |
| 12 | 2999999999991 |
| 13 | 29999999999991 |
| 14 | 299999999999991 |
| 15 | 2999999999999991 |
| 16 | 29999999999999991 |
| 17 | 299999999999999991 |
| 18 | 2999999999999999991 |
| 19 | 29999999999999999991 |
| 20 | 299999999999999999991 |