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| n | a(n) |
|---|---|
| 1 | 0 |
| 2 | 5 |
| 3 | 20 |
| 4 | 90 |
| 5 | 300 |
| 6 | 930 |
| 7 | 2610 |
| 8 | 6900 |
| 9 | 17295 |
| 10 | 41605 |
| 11 | 96660 |
| 12 | 218145 |
| 13 | 480225 |
| 14 | 1034765 |
| 15 | 2188385 |
| 16 | 4552745 |
| 17 | 9334760 |
| 18 | 18892805 |
| 19 | 37794765 |
| 20 | 74817520 |
| 21 | 146702410 |
| 22 | 285169310 |
| 23 | 549948760 |
| 24 | 1052879110 |
| 25 | 2002263910 |
| 26 | 3784182685 |
| 27 | 7110957850 |
| 28 | 13291250220 |
[0,5,20,90,300,930,2610,6900,17295,41605,96660, 218145,480225,1034765,2188385,4552745,9334760, 18892805,37794765,74817520,146702410,285169310, 549948760,1052879110,2002263910,3784182685, 7110957850,13291250220] |