| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 4 |
| 2 | 6 |
| 3 | 20 |
| 4 | 60 |
| 5 | 204 |
| 6 | 670 |
| 7 | 2340 |
| 8 | 8160 |
| 9 | 29120 |
| 10 | 104754 |
| 11 | 381300 |
| 12 | 1397740 |
| 13 | 5162220 |
| 14 | 19172790 |
| 15 | 71582716 |
| 16 | 268431360 |
| 17 | 1010580540 |
| 18 | 3817733920 |
| 19 | 14467258260 |
| 20 | 54975528948 |
| 21 | 209430785460 |
| 22 | 799644629550 |
| 23 | 3059510616420 |
[1,4,6,20,60,204,670,2340,8160,29120,104754, 381300,1397740,5162220,19172790,71582716, 268431360,1010580540,3817733920,14467258260, 54975528948,209430785460,799644629550, 3059510616420] |