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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 4 |
| 2 | 18 |
| 3 | 114 |
| 4 | 900 |
| 5 | 8845 |
| 6 | 103861 |
| 7 | 1427122 |
| 8 | 22486706 |
| 9 | 399906140 |
| 10 | 7922936720 |
| 11 | 173013117604 |
| 12 | 4127746294408 |
| 13 | 106806183646594 |
| 14 | 2978731438384738 |
| 15 | 89065499057526433 |
| 16 | 2842061902985159593 |
| 17 | 96395720127638538076 |
| 18 | 3462922846509648162418 |
[1,4,18,114,900,8845,103861,1427122,22486706, 399906140,7922936720,173013117604,4127746294408, 106806183646594,2978731438384738, 89065499057526433,2842061902985159593, 96395720127638538076,3462922846509648162418] |