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 | 1 |
| 2 | -4 |
| 3 | 21 |
| 4 | -104 |
| 5 | 521 |
| 6 | -2604 |
| 7 | 13021 |
| 8 | -65104 |
| 9 | 325521 |
| 10 | -1627604 |
| 11 | 8138021 |
| 12 | -40690104 |
| 13 | 203450521 |
| 14 | -1017252604 |
| 15 | 5086263021 |
| 16 | -25431315104 |
| 17 | 127156575521 |
| 18 | -635782877604 |
| 19 | 3178914388021 |
| 20 | -15894571940104 |
| 21 | 79472859700521 |