Anthony Trollope (2015). “The Palliser Novels: Complete Parliamentary Chronicles (All Six Novels in One Volume): Can You Forgive Her? + Phineas Finn + The Eustace Diamonds + Phineas Redux + The Prime Minister + The Duke’s Children”, p.1776, e-artnow
"A method for finding curved lines enjoying properties of maximum or minimum, or solution of isoperimetric problems in the broadest accepted sense". Book by Leonhard Euler, 1744.