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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 4 |
| 2 | 30 |
| 3 | 260 |
| 4 | 2380 |
| 5 | 22428 |
| 6 | 215292 |
| 7 | 2093520 |
| 8 | 20553390 |
| 9 | 203280220 |
| 10 | 2022339176 |
| 11 | 20215564824 |
| 12 | 202879303900 |
| 13 | 2042865050800 |
| 14 | 20629119101400 |
| 15 | 208829908532880 |
| 16 | 2118554718825420 |
| 17 | 21533269718832300 |
| 18 | 219235457827640100 |
| 19 | 2235446059461106800 |
| 20 | 22824647678376163620 |
| 21 | 233331794241184490280 |
[1,4,30,260,2380,22428,215292,2093520,20553390, 203280220,2022339176,20215564824,202879303900, 2042865050800,20629119101400,208829908532880, 2118554718825420,21533269718832300, 219235457827640100,2235446059461106800, 22824647678376163620,233331794241184490280] |