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 | 0 |
| 2 | 1 |
| 3 | 3 |
| 4 | 10 |
| 5 | 30 |
| 6 | 90 |
| 7 | 270 |
| 8 | 810 |
| 9 | 2430 |
| 10 | 7290 |
| 11 | 21870 |
| 12 | 65610 |
| 13 | 196830 |
| 14 | 590490 |
| 15 | 1771470 |
| 16 | 5314410 |
| 17 | 15943230 |
| 18 | 47829690 |
| 19 | 143489070 |
| 20 | 430467210 |
| 21 | 1291401630 |
| 22 | 3874204890 |
| 23 | 11622614670 |
| 24 | 34867844010 |
| 25 | 104603532030 |
| 26 | 313810596090 |
| 27 | 941431788270 |