The OEIS is supported by the many generous donors to the OEIS Foundation.
| n | a(n) |
|---|---|
| 1 | 1 |
| 2 | 4 |
| 3 | 20 |
| 4 | 110 |
| 5 | 608 |
| 6 | 3409 |
| 7 | 19304 |
| 8 | 110187 |
| 9 | 632960 |
| 10 | 3654915 |
| 11 | 21195716 |
| 12 | 123365099 |
| 13 | 720240584 |
| 14 | 4216176575 |
| 15 | 24738092400 |
| 16 | 145444454754 |
| 17 | 856662911136 |
| 18 | 5053815390148 |
| 19 | 29857490333880 |
| 20 | 176624209891024 |
[1,4,20,110,608,3409,19304,110187,632960,3654915, 21195716,123365099,720240584,4216176575, 24738092400,145444454754,856662911136, 5053815390148,29857490333880,176624209891024] |