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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 4 |
| 2 | 18 |
| 3 | 88 |
| 4 | 454 |
| 5 | 2424 |
| 6 | 13236 |
| 7 | 73392 |
| 8 | 411462 |
| 9 | 2325976 |
| 10 | 13233628 |
| 11 | 75682512 |
| 12 | 434662684 |
| 13 | 2505229744 |
| 14 | 14482673832 |
| 15 | 83940771168 |
| 16 | 487610895942 |
| 17 | 2838118247064 |
| 18 | 16547996212044 |
| 19 | 96635257790352 |
| 20 | 565107853947444 |
| 21 | 3308820294176016 |
| 22 | 19395905063796312 |
| 23 | 113814537122646432 |
[1,4,18,88,454,2424,13236,73392,411462,2325976, 13233628,75682512,434662684,2505229744, 14482673832,83940771168,487610895942, 2838118247064,16547996212044,96635257790352, 565107853947444,3308820294176016, 19395905063796312,113814537122646432] |