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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 1 |
| 2 | 3 |
| 3 | 18 |
| 4 | 168 |
| 5 | 2142 |
| 6 | 34704 |
| 7 | 682740 |
| 8 | 15810372 |
| 9 | 421339176 |
| 10 | 12702393792 |
| 11 | 427435993512 |
| 12 | 15881634963216 |
| 13 | 645804320863680 |
| 14 | 28527455317884336 |
| 15 | 1360332028008819360 |
| 16 | 69645942884911181184 |
| 17 | 3810436222004101378656 |
| 18 | 221867131720533800409216 |
| 19 | 13698420738298341356760768 |
[1,1,3,18,168,2142,34704,682740,15810372, 421339176,12702393792,427435993512,15881634963216, 645804320863680,28527455317884336, 1360332028008819360,69645942884911181184, 3810436222004101378656,221867131720533800409216, 13698420738298341356760768] |