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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 4 |
| 2 | 27 |
| 3 | 220 |
| 4 | 1984 |
| 5 | 19064 |
| 6 | 191325 |
| 7 | 1981932 |
| 8 | 21031965 |
| 9 | 227463808 |
| 10 | 2498039219 |
| 11 | 27782561352 |
| 12 | 312281382836 |
| 13 | 3541879743840 |
| 14 | 40484779373060 |
| 15 | 465888833819532 |
| 16 | 5393215780225983 |
| 17 | 62761359573224612 |
| 18 | 733784067570047400 |
| 19 | 8615217370731224160 |
| 20 | 101533102164551821896 |
[1,4,27,220,1984,19064,191325,1981932,21031965, 227463808,2498039219,27782561352,312281382836, 3541879743840,40484779373060,465888833819532, 5393215780225983,62761359573224612, 733784067570047400,8615217370731224160, 101533102164551821896] |