| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 1 |
| 2 | 4 |
| 3 | 20 |
| 4 | 136 |
| 5 | 1016 |
| 6 | 10112 |
| 7 | 102080 |
| 8 | 1259648 |
| 9 | 16501888 |
| 10 | 243214592 |
| 11 | 3792156928 |
| 12 | 66314635264 |
| 13 | 1201731751936 |
| 14 | 23824296632320 |
| 15 | 496708324364288 |
| 16 | 11065302289285120 |
| 17 | 257749924759273472 |
| 18 | 6397599337673523200 |
| 19 | 165009476729535463424 |
| 20 | 4496775223731602063360 |
[1,1,4,20,136,1016,10112,102080,1259648,16501888, 243214592,3792156928,66314635264,1201731751936, 23824296632320,496708324364288,11065302289285120, 257749924759273472,6397599337673523200, 165009476729535463424,4496775223731602063360] |