Abstract. For X ⊂ R n let (X, E n X ) be the usual topological space induced by the nD Euclidean topological space (R n , E n ). Based on the upper limit (U -, for short) topology (resp. the lower limit (L-, for brevity) topology), after proceeding with a digitization of (X,) with a digital k-connnectivity, we obtain a digital image from the viewpoint of digital topology in a graph-theoretical approach, i.e. Rosenfeld model [25], denoted by D U (k) (X) (resp. D L(k) (X)) in the present paper. Since a Euclidean topological homotopy has some limitations of studying a digitization of (X, E n X ), the present paper establishes the so called U (k)-homotopy (resp. L(k)-homotopy) which can be used to study homotopic properties of both (X, E n X ) and D U (k) (X) (resp. both (X, E n X ) and D L(k) (X)). The goal of the paper is to study some relationships among an ordinary homotopy equivalence, a U (k)-homotopy equivalence, an L(k)-homotopy equivalence and a khomotopy equivalence. Finally, we classify (X, E n X ) in terms of a U (k)-homotopy equivalence and an L(k)-homotopy equivalence. This approach can be used to study applied topology, approximation theory and digital geometry.