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| n | a(n) |
|---|---|
| 1 | 4 |
| 2 | 6 |
| 3 | 16 |
| 4 | 45 |
| 5 | 132 |
| 6 | 404 |
| 7 | 1296 |
| 8 | 4380 |
| 9 | 15064 |
| 10 | 53622 |
| 11 | 192696 |
| 12 | 703895 |
| 13 | 2589300 |
| 14 | 9606744 |
| 15 | 35824088 |
| 16 | 134297280 |
| 17 | 505421340 |
| 18 | 1909194056 |
| 19 | 7234153416 |
| 20 | 27489073899 |
| 21 | 104717489748 |
| 22 | 399827555604 |
| 23 | 1529763696816 |
[4,6,16,45,132,404,1296,4380,15064,53622,192696, 703895,2589300,9606744,35824088,134297280, 505421340,1909194056,7234153416,27489073899, 104717489748,399827555604,1529763696816] |