| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 4 |
| 2 | 30 |
| 3 | 224 |
| 4 | 1680 |
| 5 | 12672 |
| 6 | 96096 |
| 7 | 732160 |
| 8 | 5601024 |
| 9 | 42997760 |
| 10 | 331082752 |
| 11 | 2556051456 |
| 12 | 19778969600 |
| 13 | 153363087360 |
| 14 | 1191302553600 |
| 15 | 9268801044480 |
| 16 | 72219408138240 |
| 17 | 563445537177600 |
| 18 | 4401135695953920 |
| 19 | 34414895667609600 |
| 20 | 269374774271016960 |
| 21 | 2110381254330286080 |
[1,4,30,224,1680,12672,96096,732160,5601024, 42997760,331082752,2556051456,19778969600, 153363087360,1191302553600,9268801044480, 72219408138240,563445537177600,4401135695953920, 34414895667609600,269374774271016960, 2110381254330286080] |