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| n | a(n) |
|---|---|
| 1 | 0 |
| 2 | 2 |
| 3 | 4 |
| 4 | 20 |
| 5 | 60 |
| 6 | 230 |
| 7 | 776 |
| 8 | 2792 |
| 9 | 9720 |
| 10 | 34410 |
| 11 | 120780 |
| 12 | 425788 |
| 13 | 1497716 |
| 14 | 5274190 |
| 15 | 18562320 |
| 16 | 65348560 |
| 17 | 230024944 |
| 18 | 809742418 |
| 19 | 2850375060 |
| 20 | 10033806180 |
| 21 | 35320352940 |
| 22 | 124333050422 |
| 23 | 437670231064 |
| 24 | 1540664252600 |
| 25 | 5423363437800 |
| 26 | 19091038878650 |
| 27 | 67203259647836 |
[0,2,4,20,60,230,776,2792,9720,34410,120780, 425788,1497716,5274190,18562320,65348560, 230024944,809742418,2850375060,10033806180, 35320352940,124333050422,437670231064, 1540664252600,5423363437800,19091038878650, 67203259647836] |