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| n | a(n) |
|---|---|
| 1 | 0 |
| 2 | 20 |
| 3 | 90 |
| 4 | 560 |
| 5 | 2808 |
| 6 | 13968 |
| 7 | 68362 |
| 8 | 323280 |
| 9 | 1529974 |
| 10 | 7114300 |
| 11 | 32915192 |
| 12 | 151142772 |
| 13 | 689726508 |
| 14 | 3133687480 |
| 15 | 14169310798 |
| 16 | 63838844556 |
| 17 | 286639424578 |
| 18 | 1283124275056 |
| 19 | 5728711606322 |
| 20 | 25513799290264 |
| 21 | 113382456085042 |
| 22 | 502859346935932 |
[0,20,90,560,2808,13968,68362,323280,1529974, 7114300,32915192,151142772,689726508,3133687480, 14169310798,63838844556,286639424578, 1283124275056,5728711606322,25513799290264, 113382456085042,502859346935932] |