i want to compare the analytic solution with the solution obtained with comsol

Hi , i calculate the analytic solution of the distribution of temperature in a hollow cylinder . I did the simulation in comsol , and i draw the temperature in comsol see figure 1

my problem now i want to calculate the error between the analytic solution and the simulation when i tried to define a function analytic in comsol and use it after , the comsol don't reconize it in the result section see figure 2

please help

thanks