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| n | a(n) |
|---|---|
| 0 | 0 |
| 1 | 1 |
| 2 | 18 |
| 3 | 132 |
| 4 | 680 |
| 5 | 2880 |
| 6 | 10752 |
| 7 | 36736 |
| 8 | 117504 |
| 9 | 357120 |
| 10 | 1041920 |
| 11 | 2939904 |
| 12 | 8067072 |
| 13 | 21618688 |
| 14 | 56770560 |
| 15 | 146472960 |
| 16 | 372113408 |
| 17 | 932511744 |
| 18 | 2308571136 |
| 19 | 5653135360 |
| 20 | 13707509760 |
| 21 | 32942063616 |
| 22 | 78525759488 |
| 23 | 185799278592 |
| 24 | 436627046400 |
[0,1,18,132,680,2880,10752,36736,117504,357120, 1041920,2939904,8067072,21618688,56770560, 146472960,372113408,932511744,2308571136, 5653135360,13707509760,32942063616,78525759488, 185799278592,436627046400] |