Equation Icon  ′ u ut = u + --, a ≤ x ≤ b, t ≥ 0, (7.116 ) x u(t,b) = 1, x = b, t ≥ 0, (7.117 ) b -1 u(0,x) = x , a ≤ x ≤ b, t = 0, (7.118 )