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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 1 |
| 2 | 4 |
| 3 | 20 |
| 4 | 111 |
| 5 | 657 |
| 6 | 4067 |
| 7 | 26028 |
| 8 | 170913 |
| 9 | 1145446 |
| 10 | 7804797 |
| 11 | 53911104 |
| 12 | 376669462 |
| 13 | 2657391772 |
| 14 | 18904566514 |
| 15 | 135460704648 |
| 16 | 976795422828 |
| 17 | 7082951967141 |
| 18 | 51614974500605 |
| 19 | 377798933519164 |
| 20 | 2776363089297553 |
| 21 | 20476554379564305 |
[1,1,4,20,111,657,4067,26028,170913,1145446, 7804797,53911104,376669462,2657391772,18904566514, 135460704648,976795422828,7082951967141, 51614974500605,377798933519164,2776363089297553, 20476554379564305] |