作用素
↑ (ケリー 1968, p. 10), (Halmos 1970, p. 30)
↑ Hazewinkel 2001.
↑ Yosida 1980, p. 21.
↑ Yosida 1980, p. 43.
↑ h.m. schey (2005). Div Grad Cural and All that. New York: W W Norton. ISBN 0-393-92516-1. http://www.amazon.com/Div-Grad-Curl-All-That/dp/0393925161/ref=sr_1_1?s=books&ie=UTF8&qid=1388768941&sr=1-1&keywords=div+grad+curl