已知函数,若,则实数a的值为( )
已知全集,,,,则 ( )
在①.1{0,1,2,3};②.{1}∈{0,1,2,3};③.{0,1,2,3}{0,1,2,3};④.{0}上述四个关系中,错误的个数是: ( )
设集合的真子集个数是( )