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| n | a(n) |
|---|---|
| 1 | 4 |
| 2 | 15 |
| 3 | 52 |
| 4 | 240 |
| 5 | 1121 |
| 6 | 4888 |
| 7 | 20047 |
| 8 | 77280 |
| 9 | 277441 |
| 10 | 1093425 |
| 11 | 5279468 |
| 12 | 23647519 |
| 13 | 99429196 |
| 14 | 393425745 |
| 15 | 1457109628 |
| 16 | 4968639359 |
| 17 | 24553864319 |
| 18 | 113193708472 |
| 19 | 488133974353 |
| 20 | 1980778750800 |
| 21 | 7547952442399 |
| 22 | 26710380775592 |
| 23 | 112605054449252 |
[4,15,52,240,1121,4888,20047,77280,277441,1093425, 5279468,23647519,99429196,393425745,1457109628, 4968639359,24553864319,113193708472,488133974353, 1980778750800,7547952442399,26710380775592, 112605054449252] |