Bulatov, A. A., O. Karpova, A. M. Shur, and K. Startsev. “Lower Bounds on Words Separation: Are There Short Identities in Transformation Semigroups?”. The Electronic Journal of Combinatorics, Vol. 24, no. 3, Aug. 2017, p. P3.35, doi:10.37236/6450.