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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 2 |
| 2 | 6 |
| 3 | 20 |
| 4 | 73 |
| 5 | 281 |
| 6 | 1132 |
| 7 | 4668 |
| 8 | 19721 |
| 9 | 84781 |
| 10 | 368971 |
| 11 | 1627669 |
| 12 | 7241241 |
| 13 | 32500219 |
| 14 | 146956243 |
| 15 | 668357723 |
| 16 | 3058153676 |
| 17 | 14058081823 |
| 18 | 64914531045 |
| 19 | 300967681710 |
| 20 | 1400214412893 |
| 21 | 6536451108935 |
| 22 | 30602252860171 |
| 23 | 143666262628831 |
| 24 | 676176061560358 |
| 25 | 3189725718512715 |
| 26 | 15079740230940998 |
| 27 | 71431939122794444 |
[1,2,6,20,73,281,1132,4668,19721,84781,368971, 1627669,7241241,32500219,146956243,668357723, 3058153676,14058081823,64914531045,300967681710, 1400214412893,6536451108935,30602252860171, 143666262628831,676176061560358,3189725718512715, 15079740230940998,71431939122794444] |