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EXAMPLE
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a(4)=24 is obtained from the 24 downsets on the 8th and central rank of FD(4), each containing 8 members (enumeration is arbitrary):
1 {{},{1},{2},{3},{1,2},{1,3},{2,3},{1,2,3}}
2 {{},{1},{2},{4},{1,2},{1,4},{2,4},{1,2,4}}
3 {{},{1},{3},{4},{1,3},{1,4},{3,4},{1,3,4}}
4 {{},{2},{3},{4},{2,3},{2,4},{3,4},{2,3,4}}
5 {{},{1},{2},{3},{4},{1,2},{1,3},{2,3}}
6 {{},{1},{2},{3},{4},{1,2},{1,4},{2,4}}
7 {{},{1},{2},{3},{4},{1,3},{1,4},{3,4}}
8 {{},{1},{2},{3},{4},{2,3},{2,4},{3,4}}
9 {{},{1},{2},{3},{4},{1,2},{1,3},{1,4}}
10 {{},{1},{2},{3},{4},{1,2},{2,3},{2,4}}
11 {{},{1},{2},{3},{4},{1,3},{2,3},{3,4}}
12 {{},{1},{2},{3},{4},{1,4},{2,4},{3,4}}
13 {{},{1},{2},{3},{4},{1,2},{2,3},{3,4}}
14 {{},{1},{2},{3},{4},{1,2},{1,4},{3,4}}
15 {{},{1},{2},{3},{4},{1,2},{1,4},{2,3}}
16 {{},{1},{2},{3},{4},{1,2},{1,3},{3,4}}
17 {{},{1},{2},{3},{4},{1,2},{2,4},{3,4}}
18 {{},{1},{2},{3},{4},{1,2},{1,3},{2,4}}
19 {{},{1},{2},{3},{4},{1,3},{1,4},{2,3}}
20 {{},{1},{2},{3},{4},{1,3},{1,4},{2,4}}
21 {{},{1},{2},{3},{4},{1,3},{2,4},{3,4}}
22 {{},{1},{2},{3},{4},{1,3},{2,3},{2,4}}
23 {{},{1},{2},{3},{4},{1,4},{2,3},{3,4}}
24 {{},{1},{2},{3},{4},{1,4},{2,3},{2,4}}
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