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| n | a(n) |
|---|---|
| 0 | 0 |
| 1 | 1 |
| 2 | 4 |
| 3 | 30 |
| 4 | 304 |
| 5 | 3885 |
| 6 | 59976 |
| 7 | 1085973 |
| 8 | 22571200 |
| 9 | 529712667 |
| 10 | 13856212600 |
| 11 | 399773871068 |
| 12 | 12612288989664 |
| 13 | 431948624278795 |
| 14 | 15960564546516240 |
| 15 | 632898895109081310 |
| 16 | 26809122466181751552 |
| 17 | 1208177444352064438155 |
| 18 | 57719104861915100554200 |
| 19 | 2913802658820378870546498 |
| 20 | 154991214138728849712151200 |
[0,1,4,30,304,3885,59976,1085973,22571200, 529712667,13856212600,399773871068,12612288989664, 431948624278795,15960564546516240, 632898895109081310,26809122466181751552, 1208177444352064438155,57719104861915100554200, 2913802658820378870546498, 154991214138728849712151200] |