http://www.jmlr.org/papers/volume8/dinuzzo07a/dinuzzo07a.pdf (Publisher version)
Dinuzzo, F., Neve, M., De Nicolao, G., & Gianazza, U. (2007). On the Representer Theorem and Equivalent Degrees of Freedom of SVR. The Journal of Machine Learning Research, 8, 2467-2495.