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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 2 |
| 2 | 6 |
| 3 | 20 |
| 4 | 72 |
| 5 | 272 |
| 6 | 1064 |
| 7 | 4272 |
| 8 | 17504 |
| 9 | 72896 |
| 10 | 307648 |
| 11 | 1312896 |
| 12 | 5655808 |
| 13 | 24562176 |
| 14 | 107419264 |
| 15 | 472675072 |
| 16 | 2091206144 |
| 17 | 9296612352 |
| 18 | 41507566592 |
| 19 | 186045061120 |
| 20 | 836830457856 |
| 21 | 3776131489792 |
| 22 | 17089399689216 |
| 23 | 77548125675520 |
| 24 | 352766964908032 |
[1,2,6,20,72,272,1064,4272,17504,72896,307648, 1312896,5655808,24562176,107419264,472675072, 2091206144,9296612352,41507566592,186045061120, 836830457856,3776131489792,17089399689216, 77548125675520,352766964908032] |