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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 1 |
| 2 | 4 |
| 3 | 18 |
| 4 | 96 |
| 5 | 551 |
| 6 | 3332 |
| 7 | 20906 |
| 8 | 134820 |
| 9 | 888151 |
| 10 | 5951096 |
| 11 | 40432550 |
| 12 | 277892604 |
| 13 | 1928668910 |
| 14 | 13497833600 |
| 15 | 95150192558 |
| 16 | 674993798716 |
| 17 | 4815149310441 |
| 18 | 34519885929860 |
| 19 | 248571425473698 |
| 20 | 1797058507267104 |
| 21 | 13038781500215352 |
| 22 | 94914559729835580 |
| 23 | 692987915940266152 |
[1,1,4,18,96,551,3332,20906,134820,888151,5951096, 40432550,277892604,1928668910,13497833600, 95150192558,674993798716,4815149310441, 34519885929860,248571425473698,1797058507267104, 13038781500215352,94914559729835580, 692987915940266152] |