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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 4 |
| 2 | 28 |
| 3 | 232 |
| 4 | 2092 |
| 5 | 19884 |
| 6 | 196096 |
| 7 | 1988424 |
| 8 | 20611116 |
| 9 | 217526524 |
| 10 | 2330681348 |
| 11 | 25296553088 |
| 12 | 277653104800 |
| 13 | 3077568629256 |
| 14 | 34410056828392 |
| 15 | 387725845018512 |
| 16 | 4399241841920428 |
| 17 | 50228061806093020 |
| 18 | 576729989899675348 |
[1,4,28,232,2092,19884,196096,1988424,20611116, 217526524,2330681348,25296553088,277653104800, 3077568629256,34410056828392,387725845018512, 4399241841920428,50228061806093020, 576729989899675348] |