数学附录 14等值集,上值集,下值集: assume that f be defined on XCR, XEX, and f(x)=y. The level set, the superior set, and the inferior set for level y are respectively, the sets (y)={x∈X:f(x)=y};S(y)={x∈X:f(x)≥y;Iy {x∈x:f(x)y"}数学附录 • 14 等值集，上值集，下值集： Assume that f be defined on Xn , x 0X, and f(x0 ) = y 0 . The level set, the superior set, and the inferior set for level y 0 are, respectively, the sets: L(y0 ) = {xX: f(x)=y0 }; S(y0 ) = {xX: f(x)y 0 }; I(y0 ) = {xX: f(x)y 0 }