Assume that a function {{MATH_291}} is defined on an open interval I that contains a point {{MATH_292}}. Then we say that the function {{MATH_293}} has a {{CMD_294}} at {{MATH_295}} if {{MATH_296}} Notice, if a function {{MATH_297}} has a critical point at {{MATH_298}}, then the number {{MATH_299}} is inside some open interval {{MATH_300}}, and {{MATH_301}} is in the domain of {{MATH_302}}. {{CMD_303}}