isdifferentiable determines if the expression expr containing piecewise or piecewise functions such as abs, signum, max, ..., is of class $C\^$ class.  It returns either true or false.

The optional argument varparam can be passed to the function and in case expr is not of class $C^n$, varparam contains information of which $C^n$ class the function is, and a list of points where we have discontinuities in the n+1-th derivative.

$\mathrm{piecewise}\left(x<-1\&comma;-x\&comma;x<1\&comma;xx\&comma;x\right)$

$\left\{\begin{array}{cc}-x& x<\mathrm{-1}\\ x^2& x<1\\ x& \mathrm{otherwise}\end{array}\right.$

$\mathrm{isdifferentiable}\left(\&comma;x\&comma;1\right)$

$\mathrm{false}$

$\mathrm{isdifferentiable}\left(\&comma;x\&comma;2\&comma;'\mathrm{la}'\right)$

$\mathrm{false}$

$\mathrm{la}$

$0,\left\{\mathrm{-1}\&comma;1\right\}$

$\mathrm{isdifferentiable}\left(\mathrm{abs}\left(x\right)\&comma;x\&comma;2\right)$

$\mathrm{false}$

$\mathrm{isdifferentiable}\left(\mathrm{abs}\left(x\right)\&comma;x\&comma;2\&comma;'\mathrm{la}'\right)$

$\mathrm{false}$

$\mathrm{la}$

$0,\left\{0\right\}$