  Motivated by the Poincare conjecture, we study properties of digital
n-dimensional spheres and disks, which are digital models of their continuous
counterparts. We introduce homeomorphic transformations of digital manifolds,
which retain the connectedness, the dimension, the Euler characteristics and
the homology groups of manifolds. We find conditions where an n-dimensional
digital manifold is the n-dimensional digital sphere and discuss the link
between continuous closed n-manifolds and their digital models.
