Convexity and smoothness

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Question: What do you mean the following concepts? and when to use?
Let ##\delta _{X}:[0,2]\longrightarrow \lbrack 0,1]## }be the
modulus of convexity of X, and defined by
##\delta _{X}(\varepsilon )=\inf \{1-\frac{1}{2}\left\Vert x-y\right\Vert
:\left\Vert x\right\Vert \leq 1, \left\Vert y\right\Vert \leq 1, \left\Vert x-y\right\Vert \geq \varepsilon \}.##
And let ##\rho _{X}:[0,\infty \lbrack \longrightarrow \lbrack 0,\infty \lbrack##
be the modulus of smoothness of X, and defined by
##\rho _{X}(t) =\sup \{\frac{1}{2}(\left\Vert x+y\right\Vert +\left\Vert
x-y\right\Vert )-1:\left\Vert x\right\Vert =1,\left\Vert
y\right\Vert =t\} ##