  We study Hessian fully nonlinear uniformly elliptic equations and show that
the second derivatives of viscosity solutions of those equations (in 12 or more
dimensions) can blow up in an interior point of the domain. We prove that the
optimal interior regularity of such solutions is no more than C^{1+\epsilon},
showing the optimality of the known interior regularity result. The same is
proven for Isaacs equations. We prove the existence of non-smooth solutions to
fully nonlinear Hessian uniformly elliptic equations in 11 dimensions. We study
also the possible singularity of solutions of Hessian equations defined in a
neighborhood of a point and prove that a homogeneous order 0<\alpha<1 solution
of a Hessian uniformly elliptic equation in a punctured ball should be radial.
