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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 1 |
| 2 | 4 |
| 3 | 20 |
| 4 | 114 |
| 5 | 712 |
| 6 | 4772 |
| 7 | 33896 |
| 8 | 253102 |
| 9 | 1975610 |
| 10 | 16054568 |
| 11 | 135413280 |
| 12 | 1182664740 |
| 13 | 10675334958 |
| 14 | 99437919664 |
| 15 | 954581258020 |
| 16 | 9433732288486 |
| 17 | 95883201181772 |
| 18 | 1001411775057322 |
| 19 | 10738668800872594 |
| 20 | 118151145186400408 |
[1,1,4,20,114,712,4772,33896,253102,1975610, 16054568,135413280,1182664740,10675334958, 99437919664,954581258020,9433732288486, 95883201181772,1001411775057322,10738668800872594, 118151145186400408] |