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| n | a(n) |
|---|---|
| 1 | 0 |
| 2 | 2 |
| 3 | 6 |
| 4 | 20 |
| 5 | 72 |
| 6 | 274 |
| 7 | 1086 |
| 8 | 4438 |
| 9 | 18570 |
| 10 | 79174 |
| 11 | 342738 |
| 12 | 1502472 |
| 13 | 6656436 |
| 14 | 29756910 |
| 15 | 134061570 |
| 16 | 608072340 |
| 17 | 2774495160 |
| 18 | 12726088630 |
| 19 | 58646299650 |
| 20 | 271401086380 |
| 21 | 1260750482760 |
| 22 | 5876782098790 |
| 23 | 27479558368170 |
| 24 | 128861594138750 |
| 25 | 605869334122602 |
| 26 | 2855527261156394 |
| 27 | 13488568550452446 |
[0,2,6,20,72,274,1086,4438,18570,79174,342738, 1502472,6656436,29756910,134061570,608072340, 2774495160,12726088630,58646299650,271401086380, 1260750482760,5876782098790,27479558368170, 128861594138750,605869334122602,2855527261156394, 13488568550452446] |