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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 1 |
| 2 | 4 |
| 3 | 18 |
| 4 | 88 |
| 5 | 450 |
| 6 | 2364 |
| 7 | 12642 |
| 8 | 68464 |
| 9 | 374274 |
| 10 | 2060980 |
| 11 | 11414898 |
| 12 | 63521352 |
| 13 | 354870594 |
| 14 | 1989102444 |
| 15 | 11180805570 |
| 16 | 63001648608 |
| 17 | 355761664002 |
| 18 | 2012724468324 |
| 19 | 11406058224594 |
| 20 | 64734486343480 |
| 21 | 367891005738690 |
| 22 | 2093292414443164 |
| 23 | 11923933134635298 |
| 24 | 67990160422313808 |
[1,1,4,18,88,450,2364,12642,68464,374274,2060980, 11414898,63521352,354870594,1989102444, 11180805570,63001648608,355761664002, 2012724468324,11406058224594,64734486343480, 367891005738690,2093292414443164, 11923933134635298,67990160422313808] |