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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 4 |
| 2 | 13 |
| 3 | 39 |
| 4 | 120 |
| 5 | 407 |
| 6 | 1578 |
| 7 | 7042 |
| 8 | 35840 |
| 9 | 205253 |
| 10 | 1306454 |
| 11 | 9148392 |
| 12 | 69887664 |
| 13 | 578392583 |
| 14 | 5155022894 |
| 15 | 49226836114 |
| 16 | 501420422112 |
| 17 | 5426640606697 |
| 18 | 62184720675718 |
| 19 | 752172431553308 |
| 20 | 9576956842743904 |
| 21 | 128034481788227195 |
[1,4,13,39,120,407,1578,7042,35840,205253,1306454, 9148392,69887664,578392583,5155022894,49226836114, 501420422112,5426640606697,62184720675718, 752172431553308,9576956842743904, 128034481788227195] |