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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 1 |
| 2 | 4 |
| 3 | 12 |
| 4 | 53 |
| 5 | 237 |
| 6 | 1306 |
| 7 | 7537 |
| 8 | 47913 |
| 9 | 322253 |
| 10 | 2297874 |
| 11 | 17191216 |
| 12 | 134505656 |
| 13 | 1095715055 |
| 14 | 9267223594 |
| 15 | 81162609328 |
| 16 | 734511656413 |
| 17 | 6856030049629 |
| 18 | 65899370570285 |
| 19 | 651338242941020 |
| 20 | 6611459646337423 |
| 21 | 68842439737228261 |
[1,1,4,12,53,237,1306,7537,47913,322253,2297874, 17191216,134505656,1095715055,9267223594, 81162609328,734511656413,6856030049629, 65899370570285,651338242941020,6611459646337423, 68842439737228261] |