Fast blow-up mechanisms for sign-changing solutions of a semilinear parabolic equation with critical nonlinearity

We consider the semilinear heat equation with critical power nonlinearity. Using formal. arguments based on matched asymptotic expansion techniques, we give a detailed description of radially symmetric sign-changing solutions, which blow-up at x = 0 and t = T < <infinity>, for space dimension N = 3,4,5,6. These solutions exhibit fast blow-up; i.e. they satisfy lim(t up arrowT)(T - t)(1/(p-1))u(0, t) = infinity. In contrast, radial solutions that are positive and decreasing behave as in the subcritical case for any N greater than or equal to 3. This last result is extended in the case of exponential nonlinearity and N = 2.