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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 1 |
| 2 | 4 |
| 3 | 16 |
| 4 | 77 |
| 5 | 460 |
| 6 | 3287 |
| 7 | 27561 |
| 8 | 265307 |
| 9 | 2880875 |
| 10 | 34821316 |
| 11 | 463543454 |
| 12 | 6737545832 |
| 13 | 106158368798 |
| 14 | 1802204594518 |
| 15 | 32793160634292 |
| 16 | 636683459975767 |
| 17 | 13137118248246982 |
| 18 | 287070448575006268 |
| 19 | 6622644707103106925 |
| 20 | 160846900060253917905 |
| 21 | 4102379491083664461080 |
[1,1,4,16,77,460,3287,27561,265307,2880875, 34821316,463543454,6737545832,106158368798, 1802204594518,32793160634292,636683459975767, 13137118248246982,287070448575006268, 6622644707103106925,160846900060253917905, 4102379491083664461080] |