Let X and Z be topological spaces and Y be a subspace of X. A con-
tinuous function f : Y ! Z is extendable to X if there exists a continuous
function f : X ! Z such that f(x) = f(x) for all x 2 Y . If every continuous
function f : Y ! Z is...
In this paper Euclidean and hyperbolic discontinuous isometries* are discussed and compared. The graphic artist M.C. Escher (1898-1972) produced works which have for the structure points, lines, and angles, which repeat in the plane (Euclidean and...