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| n | a(n) |
|---|---|
| 1 | 4 |
| 2 | 20 |
| 3 | 105 |
| 4 | 576 |
| 5 | 3383 |
| 6 | 20393 |
| 7 | 121774 |
| 8 | 709169 |
| 9 | 4121638 |
| 10 | 24622476 |
| 11 | 150895459 |
| 12 | 928855881 |
| 13 | 5659650136 |
| 14 | 34105862488 |
| 15 | 205228295685 |
| 16 | 1249619101830 |
| 17 | 7761595614134 |
| 18 | 48955797807546 |
| 19 | 308737202532449 |
| 20 | 1916725035118260 |
[4,20,105,576,3383,20393,121774,709169,4121638, 24622476,150895459,928855881,5659650136, 34105862488,205228295685,1249619101830, 7761595614134,48955797807546,308737202532449, 1916725035118260] |