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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 2 |
| 2 | 4 |
| 3 | 20 |
| 4 | 144 |
| 5 | 1332 |
| 6 | 15920 |
| 7 | 225332 |
| 8 | 3758272 |
| 9 | 71711540 |
| 10 | 1544139216 |
| 11 | 37040248500 |
| 12 | 979378764320 |
| 13 | 28308318200372 |
| 14 | 887957701803952 |
| 15 | 30043664101434164 |
| 16 | 1090686549233837952 |
| 17 | 42290355849577306932 |
| 18 | 1744321111108101722768 |
| 19 | 76261355010301941319604 |
[1,2,4,20,144,1332,15920,225332,3758272,71711540, 1544139216,37040248500,979378764320, 28308318200372,887957701803952,30043664101434164, 1090686549233837952,42290355849577306932, 1744321111108101722768,76261355010301941319604] |