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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 4 |
| 2 | 12 |
| 3 | 44 |
| 4 | 172 |
| 5 | 772 |
| 6 | 3308 |
| 7 | 14924 |
| 8 | 64956 |
| 9 | 294252 |
| 10 | 1301044 |
| 11 | 5930588 |
| 12 | 26506948 |
| 13 | 121290940 |
| 14 | 546050988 |
| 15 | 2505533940 |
| 16 | 11340303508 |
| 17 | 52147596788 |
| 18 | 236995050900 |
| 19 | 1091701675948 |
| 20 | 4977541017540 |
| 21 | 22961416861940 |
| 22 | 104965762062612 |
[1,4,12,44,172,772,3308,14924,64956,294252, 1301044,5930588,26506948,121290940,546050988, 2505533940,11340303508,52147596788,236995050900, 1091701675948,4977541017540,22961416861940, 104965762062612] |