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| n | a(n) |
|---|---|
| 0 | 0 |
| 1 | 1 |
| 2 | 4 |
| 3 | 18 |
| 4 | 80 |
| 5 | 350 |
| 6 | 1512 |
| 7 | 6468 |
| 8 | 27456 |
| 9 | 115830 |
| 10 | 486200 |
| 11 | 2032316 |
| 12 | 8465184 |
| 13 | 35154028 |
| 14 | 145608400 |
| 15 | 601749000 |
| 16 | 2481880320 |
| 17 | 10218366630 |
| 18 | 42004911960 |
| 19 | 172427570700 |
| 20 | 706905276000 |
| 21 | 2894777105220 |
| 22 | 11841673237680 |
| 23 | 48394276165560 |
| 24 | 197602337462400 |
| 25 | 806190092077500 |
[0,1,4,18,80,350,1512,6468,27456,115830,486200, 2032316,8465184,35154028,145608400,601749000, 2481880320,10218366630,42004911960,172427570700, 706905276000,2894777105220,11841673237680, 48394276165560,197602337462400,806190092077500] |