As I understand it, searching for
title: "halting problem should return only questions with the quoted string in the title. However, that is not the case, and indeed those results are the same as searching for
It appears that I need to remove the space after "title:" in order to get the desired results:
title:"halting problem". That the space made the "title:" portion useless was surprising to me. Furthermore, the Search Options page indicates that "title:" and "body:" operate on all following terms. That doesn't seem to be happening here.