XCB, the Last of the Shortest Single Axioms for the Classical   Equivalential Calculus
  It has long been an open question whether the formula XCB = EpEEEpqErqr is, with the rules of substitution and detachment, a single axiom for the classical equivalential calculus. This paper answers that question affirmatively, thus completing a search for all such eleven-symbol single axioms that began seventy years ago. 
Larry Wos, Dolph Ulrich, Branden Fitelson