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 | 7 |
| 2 | 47 |
| 3 | 0 |
| 4 | 4447 |
| 5 | 44777 |
| 6 | 0 |
| 7 | 4444747 |
| 8 | 44447747 |
| 9 | 0 |
| 10 | 4444444447 |
| 11 | 44444444747 |
| 12 | 0 |
| 13 | 4444444447777 |
| 14 | 44444447447447 |
| 15 | 0 |
| 16 | 4444444447447777 |
| 17 | 44444444444444477 |
| 18 | 0 |
| 19 | 4444444444444444777 |
| 20 | 44444444444444444447 |
| 21 | 0 |
| 22 | 4444444444444444444747 |
| 23 | 44444444444444444444477 |
| 24 | 0 |