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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 4 |
| 2 | 28 |
| 3 | 240 |
| 4 | 2288 |
| 5 | 23296 |
| 6 | 248064 |
| 7 | 2728704 |
| 8 | 30764800 |
| 9 | 353633280 |
| 10 | 4128783360 |
| 11 | 48827351040 |
| 12 | 583674642432 |
| 13 | 7041154416640 |
| 14 | 85610725769216 |
| 15 | 1048040981594112 |
| 16 | 12907157115568128 |
| 17 | 159802897621319680 |
| 18 | 1987875305403187200 |
| 19 | 24833149969036738560 |
| 20 | 311409431144819589120 |
[1,4,28,240,2288,23296,248064,2728704,30764800, 353633280,4128783360,48827351040,583674642432, 7041154416640,85610725769216,1048040981594112, 12907157115568128,159802897621319680, 1987875305403187200,24833149969036738560, 311409431144819589120] |