Kapranov-Vasserot IV, (1.7)

Vladimir Drinfeld, Infinite-dimensional vector bundles in algebraic geometry, in Pavel Etinghof, V. Retakh, Isadore Singer, The unity of mathematics, Progr. Math., 244, BirkhĂ¤user Boston (2006)

Mikhail Kapranov, E. Vasserot, Formal loops IV: Chiral differential operators (arXiv:math/0612371)