Bony, Jean-Michel, Ferruccio Colombini, and Ludovico Pernazza. 2010. “On Square Roots of Class C^m Of Nonnegative Functions of One Variable”. ANNALI SCUOLA NORMALE SUPERIORE - CLASSE DI SCIENZE 9 (3):635-44. https://doi.org/10.2422/2036-2145.2010.3.08.