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| n | a(n) |
|---|---|
| 0 | 0 |
| 1 | 1 |
| 2 | 4 |
| 3 | 18 |
| 4 | 80 |
| 5 | 356 |
| 6 | 1584 |
| 7 | 7048 |
| 8 | 31360 |
| 9 | 139536 |
| 10 | 620864 |
| 11 | 2762528 |
| 12 | 12291840 |
| 13 | 54692416 |
| 14 | 243353344 |
| 15 | 1082798208 |
| 16 | 4817899520 |
| 17 | 21437194496 |
| 18 | 95384577024 |
| 19 | 424412697088 |
| 20 | 1888419942400 |
| 21 | 8402505163776 |
| 22 | 37386860539904 |
| 23 | 166352452487168 |
[0,1,4,18,80,356,1584,7048,31360,139536,620864, 2762528,12291840,54692416,243353344,1082798208, 4817899520,21437194496,95384577024,424412697088, 1888419942400,8402505163776,37386860539904, 166352452487168] |