@misc{Cayley_Arthur_(1821–1895)_Demonstration_1889,
 author={Cayley Arthur (1821–1895)},
 copyright={Public Domain},
 journal={Mathematical papers},
 address={Cambridge},
 howpublished={online},
 year={1889},
 publisher={Cambridge University Press},
 language={eng},
 type={Text},
 title={Demonstration of a Theorem relating to the Products of Sums of Squares},
 URL={http://www.rcin.org.pl/Content/122048/PDF/WA35_149106_12807-2_Art12.pdf},
}