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| n | a(n) |
|---|---|
| 0 | 0 |
| 1 | 2 |
| 2 | 4 |
| 3 | 20 |
| 4 | 64 |
| 5 | 248 |
| 6 | 880 |
| 7 | 3248 |
| 8 | 11776 |
| 9 | 43040 |
| 10 | 156736 |
| 11 | 571712 |
| 12 | 2083840 |
| 13 | 7597952 |
| 14 | 27698944 |
| 15 | 100985600 |
| 16 | 368164864 |
| 17 | 1342243328 |
| 18 | 4893475840 |
| 19 | 17840411648 |
| 20 | 65041678336 |
| 21 | 237125826560 |
| 22 | 864501723136 |
| 23 | 3151758405632 |
| 24 | 11490527150080 |
| 25 | 41891604733952 |
| 26 | 152726372368384 |
[0,2,4,20,64,248,880,3248,11776,43040,156736, 571712,2083840,7597952,27698944,100985600, 368164864,1342243328,4893475840,17840411648, 65041678336,237125826560,864501723136, 3151758405632,11490527150080,41891604733952, 152726372368384] |