Cajori, Florian (1993). A History of Mathematical Notation. Courier Dover Publications. pp. 382–383. ISBN9780486677668. Consultado el 11 de octubre de 2012. «Nor is there historical evidence to support the statement found in Noah Webster's Dictionary, under the letter x, to the effect that 'x was used as an abbreviation of Ar. shei (a thing), something, which, in the Middle Ages, was used to designate the unknown, and was then prevailingly transcribed as xei.'».
