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| n | a(n) |
|---|---|
| 0 | 0 |
| 1 | 1 |
| 2 | 1 |
| 3 | 1 |
| 4 | 1 |
| 5 | 1 |
| 6 | 2 |
| 7 | 4 |
| 8 | 12 |
| 9 | 20 |
| 10 | 140 |
| 11 | 2380 |
| 12 | 30940 |
| 13 | 4053140 |
| 14 | 976806740 |
| 15 | 1800254821820 |
| 16 | 61120451455610820 |
| 17 | 9679503282638355804820 |
| 18 | 644895289728732254890713095060 |
| 19 | 11032242225369970870930018496422256556580 |
| 20 | 123023889999524997120705110893674654359714272362614340 |
[0,1,1,1,1,1,2,4,12,20,140,2380,30940,4053140, 976806740,1800254821820,61120451455610820, 9679503282638355804820, 644895289728732254890713095060, 11032242225369970870930018496422256556580, 12302388999952499712070511089367465435971427236261\ 4340] |