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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 1 |
| 2 | 2 |
| 3 | 6 |
| 4 | 28 |
| 5 | 160 |
| 6 | 1080 |
| 7 | 8470 |
| 8 | 76160 |
| 9 | 771120 |
| 10 | 8671600 |
| 11 | 107245600 |
| 12 | 1446984000 |
| 13 | 21150929800 |
| 14 | 332950217600 |
| 15 | 5615507898000 |
| 16 | 101024594070400 |
| 17 | 1931055071545600 |
| 18 | 39082823446867200 |
| 19 | 834945681049480000 |
| 20 | 18776164188349568000 |
| 21 | 443348081412556320000 |
[1,1,2,6,28,160,1080,8470,76160,771120,8671600, 107245600,1446984000,21150929800,332950217600, 5615507898000,101024594070400,1931055071545600, 39082823446867200,834945681049480000, 18776164188349568000,443348081412556320000] |