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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 4 |
| 2 | 18 |
| 3 | 82 |
| 4 | 374 |
| 5 | 1704 |
| 6 | 7752 |
| 7 | 35214 |
| 8 | 159750 |
| 9 | 723880 |
| 10 | 3276908 |
| 11 | 14821668 |
| 12 | 66991436 |
| 13 | 302605528 |
| 14 | 1366182276 |
| 15 | 6165204102 |
| 16 | 27811282374 |
| 17 | 125415953208 |
| 18 | 565408947756 |
| 19 | 2548400193852 |
| 20 | 11483706241044 |
| 21 | 51739037228688 |
| 22 | 233070330199296 |
[1,4,18,82,374,1704,7752,35214,159750,723880, 3276908,14821668,66991436,302605528,1366182276, 6165204102,27811282374,125415953208,565408947756, 2548400193852,11483706241044,51739037228688, 233070330199296] |