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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 3 |
| 2 | 18 |
| 3 | 140 |
| 4 | 1260 |
| 5 | 12474 |
| 6 | 132132 |
| 7 | 1472328 |
| 8 | 17065620 |
| 9 | 204155380 |
| 10 | 2506399896 |
| 11 | 31443925968 |
| 12 | 401783498480 |
| 13 | 5215458874500 |
| 14 | 68633685693000 |
| 15 | 914099013896400 |
| 16 | 12304253831789700 |
| 17 | 167193096184907100 |
| 18 | 2291164651422801000 |
| 19 | 31637804708163654000 |
| 20 | 439903041116118980400 |
[1,3,18,140,1260,12474,132132,1472328,17065620, 204155380,2506399896,31443925968,401783498480, 5215458874500,68633685693000,914099013896400, 12304253831789700,167193096184907100, 2291164651422801000,31637804708163654000, 439903041116118980400] |