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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 4 |
| 2 | 18 |
| 3 | 88 |
| 4 | 452 |
| 5 | 2384 |
| 6 | 12744 |
| 7 | 68576 |
| 8 | 370192 |
| 9 | 2001472 |
| 10 | 10829088 |
| 11 | 58612096 |
| 12 | 317289536 |
| 13 | 1717746944 |
| 14 | 9299922048 |
| 15 | 50350919168 |
| 16 | 272608444672 |
| 17 | 1475954689024 |
| 18 | 7991119286784 |
| 19 | 43265588647936 |
| 20 | 234249039168512 |
| 21 | 1268274072276992 |
[1,4,18,88,452,2384,12744,68576,370192,2001472, 10829088,58612096,317289536,1717746944,9299922048, 50350919168,272608444672,1475954689024, 7991119286784,43265588647936,234249039168512, 1268274072276992] |