XCB, the Last of the Shortest Single Axioms for the Classical Equivalential Calculus
<|reference_start|>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.<|reference_end|>
@article{wos2002xcb,,
    title={XCB, the Last of the Shortest Single Axioms for the Classical
  Equivalential Calculus},
    author={Larry Wos, Dolph Ulrich, Branden Fitelson},
    journal={arXiv preprint arXiv:cs/0211015},
    year={2002},
    number={ANL/MCS-P966-0602},
    archivePrefix={arXiv},
    eprint={cs/0211015},
    primaryClass={cs.LO cs.AI}
}