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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 4 |
| 2 | 15 |
| 3 | 56 |
| 4 | 208 |
| 5 | 767 |
| 6 | 2812 |
| 7 | 10278 |
| 8 | 37530 |
| 9 | 137044 |
| 10 | 500571 |
| 11 | 1828818 |
| 12 | 6682264 |
| 13 | 24416877 |
| 14 | 89218462 |
| 15 | 325997507 |
| 16 | 1191160160 |
| 17 | 4352355633 |
| 18 | 15902968338 |
| 19 | 58107491971 |
| 20 | 212317732888 |
| 21 | 775783501558 |
| 22 | 2834620130881 |
| 23 | 10357363200392 |
| 24 | 37844566834330 |
| 25 | 138279520124262 |
[1,4,15,56,208,767,2812,10278,37530,137044,500571, 1828818,6682264,24416877,89218462,325997507, 1191160160,4352355633,15902968338,58107491971, 212317732888,775783501558,2834620130881, 10357363200392,37844566834330,138279520124262] |