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| n | a(n) |
|---|---|
| 2 | 4 |
| 3 | 15 |
| 4 | 112 |
| 5 | 440 |
| 6 | 3328 |
| 7 | 13552 |
| 8 | 103168 |
| 9 | 429696 |
| 10 | 3282944 |
| 11 | 13882880 |
| 12 | 106307584 |
| 13 | 454471680 |
| 14 | 3485499392 |
| 15 | 15022878720 |
| 16 | 115345653760 |
| 17 | 500317061120 |
| 18 | 3844727111680 |
| 19 | 16761479102464 |
| 20 | 128891635105792 |
| 21 | 564250624917504 |
| 22 | 4341301000536064 |
[4,15,112,440,3328,13552,103168,429696,3282944, 13882880,106307584,454471680,3485499392, 15022878720,115345653760,500317061120, 3844727111680,16761479102464,128891635105792, 564250624917504,4341301000536064] |