Hartman and Nirenberg
showed that any C∞ isometric immersion f :En→ En+1 between flat
Euclidean spaces is a cylinder erected over a plane curve. We show that
in the codimension two case, f : En→ En+2 factors as a composition of
isometric immersions f = f1∘ f2: En→ En+1→ En+2, when n > 1 and f has
nowhere zero normal curvature. Counterexamples are given if this assumption is
relaxed.