The points in R where the function -1 tan X, f(x) = { 4 R -sgn(x) + |x-1 2 (where sgn(x) equals +1 if x>0, -1 if x < 0 and is equal to zero if x = 0), is differentiable R R-{-1} R-{1,-1} if x1 if x>1