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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 4 |
| 2 | 27 |
| 3 | 209 |
| 4 | 1716 |
| 5 | 14553 |
| 6 | 125971 |
| 7 | 1105885 |
| 8 | 9809019 |
| 9 | 87691592 |
| 10 | 788832045 |
| 11 | 7131655908 |
| 12 | 64743390321 |
| 13 | 589808771881 |
| 14 | 5389066722654 |
| 15 | 49365637128655 |
| 16 | 453212161425716 |
| 17 | 4168951499299185 |
| 18 | 38415242186255419 |
| 19 | 354527945536409116 |
| 20 | 3276414018301664025 |
[1,4,27,209,1716,14553,125971,1105885,9809019, 87691592,788832045,7131655908,64743390321, 589808771881,5389066722654,49365637128655, 453212161425716,4168951499299185, 38415242186255419,354527945536409116, 3276414018301664025] |