Buttazzo, Giaquinta & Hildebrandt 1998,Theorem 5.17 Buttazzo, Giuseppe; Giaquinta, Mariano; Hildebrandt, Stefan, One-dimensional variational problems, Oxford Lecture Series in Mathematics and its Applications 15, The Clarendon Press Oxford University Press, 1998, ISBN 978-0-19-850465-8, MR 1694383.