{a1}çåé:Ï,{a1}ã2个=2^1ã
{a1,a2,a3}çåé:Ï,{a1},{a2},{a3},{a1,a2},{a1,a3},{a2,a3},{a1,a2,a3}ã8个=2^3ã
{a1,a2,a3,a4}çåé:Ï,{a1},{a2},{a3},{a4},{a1,a2},{a1,a3},{a1,a4},{a2,a3},{a2,a4},{a3,a4},{a1,a2,a3},{a1,a2,a4},{a1,a3,a4},{a2,a3,a4},{a1,a2,a3,a4}ã16个=2^4ã
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çæ³éå{a1,a2,a3 .an}çåéç个æ°ä¸º2^n个
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