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| n | a(n) |
|---|---|
| 1 | 0 |
| 2 | 4 |
| 3 | 1 |
| 4 | 18 |
| 5 | 41 |
| 6 | 130 |
| 7 | 510 |
| 8 | 1740 |
| 9 | 5554 |
| 10 | 20089 |
| 11 | 69607 |
| 12 | 233844 |
| 13 | 815086 |
| 14 | 2824883 |
| 15 | 9656170 |
| 16 | 33351986 |
| 17 | 115270625 |
| 18 | 396350451 |
| 19 | 1366335435 |
| 20 | 4714673357 |
| 21 | 16240450509 |
| 22 | 55971672188 |
| 23 | 193001422391 |
| 24 | 665159525391 |
| 25 | 2292527902957 |
| 26 | 7903063069976 |
[0,4,1,18,41,130,510,1740,5554,20089,69607,233844, 815086,2824883,9656170,33351986,115270625, 396350451,1366335435,4714673357,16240450509, 55971672188,193001422391,665159525391, 2292527902957,7903063069976] |