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## Main Question or Discussion Point

According to supremum and infimum principle,

nonempty set A={x|x[tex]\in[/tex]Q,x

When the principle is valid?

nonempty set A={x|x[tex]\in[/tex]Q,x

^{2}<2} is upper bounded, so it should have a least upper bound. In fact, it dose not have least upper bound. Why?When the principle is valid?

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