My wife & I are undergoing a divorce. My two yeshiva educated sons wish to have nothing to do with me. What is their obligation to their father?

They have no obligation to stay in contact with their father. However, it may be a good idea to try to persuade them, by a third party who is close to them, to end the conflict in good terms - since sometimes while living separately it is easier to keep in touch, even rarely. After all, they aren't denying you're their father, and they did receive much from you over the years. Rabbi Ro'i Margalit