Should/Can I retract a paper that has fatal math error after this paper has been accepted, presented, and released in the proceeding?
For a journal paper you could either retract it, which means to "unpublish" it, or you could add an erratum, i.e. a short note where you explain what parts of the paper are wrong (and how to fix them, in case you know it) and which parts are still correct. 

If I understood your case correctly, it seems like an erratum stating that the mathematical proof is incorrect, while the experiments still support the claim, would be appropriate.