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| n | a(n) |
|---|---|
| 0 | 4 |
| 1 | 15 |
| 2 | 52 |
| 3 | 210 |
| 4 | 1007 |
| 5 | 5156 |
| 6 | 25571 |
| 7 | 133293 |
| 8 | 727082 |
| 9 | 3874464 |
| 10 | 21072166 |
| 11 | 117829671 |
| 12 | 654556778 |
[4,15,52,210,1007,5156,25571,133293,727082, 3874464,21072166,117829671,654556778] |