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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 1 |
| 2 | 4 |
| 3 | 18 |
| 4 | 132 |
| 5 | 1140 |
| 6 | 12720 |
| 7 | 164640 |
| 8 | 2514960 |
| 9 | 43500240 |
| 10 | 850076640 |
| 11 | 18418609440 |
| 12 | 439831909440 |
| 13 | 11457415569600 |
| 14 | 323707663319040 |
| 15 | 9854548934630400 |
| 16 | 321709145793235200 |
| 17 | 11209975693710393600 |
| 18 | 415330670608805952000 |
| 19 | 16303720885477254028800 |
[1,1,4,18,132,1140,12720,164640,2514960,43500240, 850076640,18418609440,439831909440,11457415569600, 323707663319040,9854548934630400, 321709145793235200,11209975693710393600, 415330670608805952000,16303720885477254028800] |