The OEIS is supported by the many generous donors to the OEIS Foundation.
| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 1 |
| 2 | 2 |
| 3 | 6 |
| 4 | 20 |
| 5 | 60 |
| 6 | 174 |
| 7 | 490 |
| 8 | 1352 |
| 9 | 3672 |
| 10 | 9850 |
| 11 | 26158 |
| 12 | 68892 |
| 13 | 180180 |
| 14 | 468454 |
| 15 | 1211730 |
| 16 | 3120400 |
| 17 | 8004144 |
| 18 | 20460402 |
| 19 | 52139990 |
| 20 | 132502180 |
| 21 | 335882988 |
| 22 | 849507230 |
| 23 | 2144114234 |
| 24 | 5401408344 |
| 25 | 13583493000 |
| 26 | 34105191146 |
| 27 | 85504030974 |
| 28 | 214070361260 |
| 29 | 535269125508 |
| 30 | 1336814464470 |
[1,1,2,6,20,60,174,490,1352,3672,9850,26158,68892, 180180,468454,1211730,3120400,8004144,20460402, 52139990,132502180,335882988,849507230,2144114234, 5401408344,13583493000,34105191146,85504030974, 214070361260,535269125508,1336814464470] |