Horseshoes in the measure-preserving Henon map

We show, using elementary methods, that for 0 < a the measure-preserving, orientation-preserving Henon map, H, has a horseshoe. This improves on the result of Devaney and Nitecki who have shown that a horseshoe exists in this map for a greater than or equal to 8. For a > 0, we also prove the conjecture of Devaney that the first symmetric homoclinic point is transversal. To obtain our results, we show that for a branch, C-u, of the unstable manifold of a hyperbolic fixed point of H, C-u crosses the line y = -x and that this crossing is a homoclinic point, chi(c). This has been shown by Devaney, but we obtain the crossing using simpler methods. Next we show that if the crossing of W-u(p) and W-s(p) at chi(c) is degenerate then the slope of C-u at this crossing is one. Following this we show that if chi(c) is a degenerate homoclinic its x-coordinate must be greater than 1/(2a). We then derive a contradiction from this by showing that the slope of C-u at H-1(chi(c)) must be both positive and negative, thus we conclude that chi(c) is transversal. Our approach uses a lemma that gives a recursive formula for the sign of curvature of the unstable manifold. This lemma, referred to as 'the curvature lemma', is the key to reducing the proof to elementary methods. A curvature lemma can be derived for a very broad array of maps making the applicability of these methods very general. Further, since curvature is the strongest differentiability feature needed in our proof, the methods work for maps of the plane which are only C-2.