a(3)=45集合系统如下({}和{1,2,3}未显示)。
{1} {1}{2} {1}{2}{3} {1}{2}{3}{12} {1}{2}{3}{12}{13}
{2} {1}{3} {1}{2}{12} {1}{2}{3}{13} {1}{2}{3}{12}{23}
{3} {2}{3} {1}{2}{13} {1}{2}{3}{23} {1}{2}{3}{13}{23}
{12} {1}{12} {1}{2}{23} {1}{2}{12}{13}
{13} {1}{13} {1}{3}{12} {1}{2}{12}{23}
{23}{1}{23}{1}{3}{13}{1}{3}{12}{13}{1}{2}{3}{12}{13}{23}
{2}{12} {1}{3}{23} {1}{3}{13}{23}
{2}{13} {2}{3}{12} {2}{3}{12}{23}
{2}{23} {2}{3}{13} {2}{3}{13}{23}
{3}{12} {2}{3}{23}
{3}{13} {1}{12}{13}
{3} {23}{2}{12}{23}
{3}{13}{23}
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