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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 1 |
| 2 | 4 |
| 3 | 18 |
| 4 | 126 |
| 5 | 1110 |
| 6 | 12120 |
| 7 | 156660 |
| 8 | 2341500 |
| 9 | 39701340 |
| 10 | 752839920 |
| 11 | 15785181720 |
| 12 | 362606123880 |
| 13 | 9055825538760 |
| 14 | 244296192460320 |
| 15 | 7079382509799600 |
| 16 | 219321853964413200 |
| 17 | 7233629128601475600 |
| 18 | 253054306933115688000 |
| 19 | 9358989706213886138400 |
| 20 | 364860828050107348159200 |
[1,1,4,18,126,1110,12120,156660,2341500,39701340, 752839920,15785181720,362606123880,9055825538760, 244296192460320,7079382509799600, 219321853964413200,7233629128601475600, 253054306933115688000,9358989706213886138400, 364860828050107348159200] |