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| n | a(n) |
|---|---|
| 1 | 4 |
| 2 | 18 |
| 3 | 117 |
| 4 | 840 |
| 5 | 6240 |
| 6 | 47139 |
| 7 | 360347 |
| 8 | 2779528 |
| 9 | 21588898 |
| 10 | 168613534 |
| 11 | 1322868729 |
| 12 | 10417760419 |
| 13 | 82302522544 |
| 14 | 651981307285 |
| 15 | 5177047812119 |
| 16 | 41193230296706 |
| 17 | 328367767613750 |
| 18 | 2621791862367316 |
| 19 | 20963552382355484 |
| 20 | 167840827307586780 |
[4,18,117,840,6240,47139,360347,2779528,21588898, 168613534,1322868729,10417760419,82302522544, 651981307285,5177047812119,41193230296706, 328367767613750,2621791862367316, 20963552382355484,167840827307586780] |