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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 4 |
| 2 | 22 |
| 3 | 152 |
| 4 | 1241 |
| 5 | 11444 |
| 6 | 115390 |
| 7 | 1243672 |
| 8 | 14104480 |
| 9 | 166460800 |
| 10 | 2028202288 |
| 11 | 25363355200 |
| 12 | 324098616925 |
| 13 | 4217387014948 |
| 14 | 55737166570870 |
| 15 | 746544123583928 |
| 16 | 10116388473816503 |
| 17 | 138496854665195996 |
| 18 | 1913322982776458234 |
| 19 | 26646647187379206440 |
| 20 | 373800949052597088329 |
[1,4,22,152,1241,11444,115390,1243672,14104480, 166460800,2028202288,25363355200,324098616925, 4217387014948,55737166570870,746544123583928, 10116388473816503,138496854665195996, 1913322982776458234,26646647187379206440, 373800949052597088329] |