We now check item {{CMD_99}} of the checklist by verifying that {{MATH_100}} exists. In order to calculate {{MATH_101}}, we have to compute two one-sided limits (since the expression for {{MATH_102}} if {{MATH_103}} is different from the expression for {{MATH_104}} if {{MATH_105}}). Looking at the limit from the left, we have {{ENV_106}} Looking at the limit from the right, we have {{ENV_107}} Hence, for the limit {{MATH_108}} to exist, we must have that {{ENV_109}} Item {{CMD_110}} is then satisfied at the point {{MATH_111}} whenever {{MATH_112}}.