足formula_3都有formula_4，则formula_5称作集合formula_6的上界，若存在formula_7，能满足formula_3都有formula_9，则formula_10称作formula_6的下界。例如在实变数中，若存在一个实数formula_12，能满足formula_13都有formula_14，则formula_12即为集合formula_16的上界，若存在一个实数formula_17，能满足formula_13都有formula_19，则formula_17即为集合fo