Global existence vs. blowup in a fully parabolic quasilinear 1D Keller–Segel system

We show that the one-dimensional fully parabolic Keller–Segel system with nonlinear diffusion possesses global-in-time solutions, provided the nonlinear diffusion is equal to 1(1+u)α,α<1, independently on the volume of the initial data. We also show that in the critical case, i.e. for α=1α=1, the same result holds for initial masses smaller than a prescribed constant. Additionally, we prove the existence of initial data for which a solution blows up in a finite time for any nonlinear diffusion integrable at infinity. However, in the parabolic–elliptic case the above mentioned integrability condition on nonlinear diffusion sharply distinguishes between global existence and blowup cases. We are unable to recover the entire global existence counterpart of this result in a fully parabolic case.