| n | a(n) |
|---|---|
| 0 | 0 |
| 1 | 4 |
| 2 | 28 |
| 3 | 224 |
| 4 | 1784 |
| 5 | 13692 |
| 6 | 102036 |
| 7 | 745432 |
| 8 | 5374192 |
| 9 | 38400500 |
| 10 | 272709644 |
| 11 | 1928498640 |
| 12 | 13597146600 |
| 13 | 95668307308 |
| 14 | 672119557252 |
| 15 | 4717043931848 |
| 16 | 33080342679008 |
| 17 | 231867574534116 |
| 18 | 1624598900644860 |
| 19 | 11379821699045056 |
[0,4,28,224,1784,13692,102036,745432,5374192, 38400500,272709644,1928498640,13597146600, 95668307308,672119557252,4717043931848, 33080342679008,231867574534116,1624598900644860, 11379821699045056] |