24
[17Y]Let \(Cβ β^ n\) be a convex set, and \(f:Cββ\) a function. \(f\) is convex if
\[ β tβ[0,1],~ ~ β x,yβ C, ~ ~ f\big(tx+(1-t)y\big)β€ tf(x)+(1-t)f(y)~ ~ . \]
\(f\) is strictly convex if also
\[ β tβ(0,1),~ ~ β x,yβ C, xβ y, ~ ~ f\big(tx+(1-t)y\big){\lt} tf(x)+(1-t)f(y)~ ~ . \]