The present writer's father, W. H. Young, used to recall that this very question — what principle can we use as the foundation of the calculus of variations[4] — had been put him by a young Italian mathematician. His reply was a question: "Can you use semicontinuity?" The young Italian was Leonida Tonelli. Semicontinuity was then still a recent concept, known only to a few. In the hands of Tonelli, it became an important tool in a fundamental new approach to the calculus of variations.
Accademia Nazionale dei Lincei (2012), Annuario dell'Accademia Nazionale dei Lincei 2012 – CDX dalla Sua Fondazione(PDF) (in Italian), Roma: Accademia Nazionale dei Lincei, p. 734, archived from the original(PDF) on 2016-03-04, retrieved 2016-05-15. The "Yearbook" of the renowned Italian scientific institution, including an historical sketch of its history, the list of all past and present members as well as a wealth of information about its academic and scientific activities.
Cesari, Lamberto (1986), "L'opera di Leonida Tonelli e la sua influenza nel pensiero scientifico del secolo", in Montalenti, G.; Amerio, L.; Acquaro, G.; Baiada, E.; Cesari, L.; Ciliberto, C.; Cimmino, G.; Cinquini, S.; De Giorgi, E.; Faedo, S.; Fichera, G.; Galligani, I.; Ghizzetti, A.; Graffi, D.; Greco, D.; Grioli, G.; Magenes, E.; Martinelli, E.; Pettineo, B.; Scorza, G.; Vesentini, E. (eds.), Convegno celebrativo del centenario della nascita di Mauro Picone e Leonida Tonelli (6–9 maggio 1985), Atti dei Convegni Lincei (in Italian), vol. 77, Roma: Accademia Nazionale dei Lincei, pp. 41–73, archived from the original on 2011-02-23, retrieved 2013-01-19. "The work of Leonida Tonelli and his influence on scientific thinking in this century" (English translation of the title) is an ample commemorative article, reporting recollections of the Author about teachers and colleagues, and a detailed survey of his and theirs scientific work, presented at the International congress in occasion of the celebration of the centenary of birth of Mauro Picone and Leonida Tonelli (held in Rome on May 6–9, 1985).
Cinquini, Silvio (1950), "Leonida Tonelli", Annali della Scuola Normale Superiore di Pisa, Classe di Scienze, Serie II (in Italian), 15 (1–4): 1–37, MR1556828, Zbl0036.00505.
Faedo, Sandro (1986), "Leonida Tonelli e la scuola matematica pisana", in Montalenti, G.; Amerio, L.; Acquaro, G.; Baiada, E.; Cesari, L.; Ciliberto, C.; Cimmino, G.; Cinquini, S.; De Giorgi, E.; Faedo, S.; Fichera, G.; Galligani, I.; Ghizzetti, A.; Graffi, D.; Greco, D.; Grioli, G.; Magenes, E.; Martinelli, E.; Pettineo, B.; Scorza, G.; Vesentini, E. (eds.), Convegno celebrativo del centenario della nascita di Mauro Picone e Leonida Tonelli (6–9 maggio 1985), Atti dei Convegni Lincei (in Italian), vol. 77, Roma: Accademia Nazionale dei Lincei, pp. 89–109, archived from the original on 2011-02-23, retrieved 2013-01-19. "Leonida Tonelli and the Pisa mathematical school" is a survey of the work of Tonelli in Pisa and his influence on the development of the school, presented at the International congress in occasion of the celebration of the centenary of birth of Mauro Picone and Leonida Tonelli (held in Rome on May 6–9, 1985). The Author was one of his pupils and, after his death, held his chair of mathematical analysis at the University of Pisa, becoming dean of the faculty of sciences and then rector: he exerted a strong positive influence on the development of the university.
Fichera, Gaetano (1995), "Tre battaglie perdute da tre grandi matematici italiani", Atti del convegno di studi in memoria di Giuseppe Gemignani. Modena, 20 maggio 1994, Collana di Studi dell'Accademia (in Italian), vol. 11, Modena: Enrico Mucchi Editore on behalf of the Accademia Nazionale di Scienze, Lettere e Arti di Modena, pp. 9–28, MR1385469((citation)): External link in |publisher= (help). This paper, included in the Proceedings of the Study Meeting in Memory of Giuseppe Gemignani, is an account of the failures of Vito Volterra, Leonida Tonelli and Francesco Severi, when dealing with particular research problems during their career. An English translation of the title reads as:-"Three battles lost by three great Italian mathematicians".
Cafiero, Federico (1959), Misura e integrazione, Monografie matematiche del Consiglio Nazionale delle Ricerche (in Italian), vol. 5, Roma: Edizioni Cremonese, pp. VII+451, MR0215954, Zbl0171.01503. Measure and integration (as the English translation of the title reads) is a definitive monograph on integration and measure theory: the treatment of the limiting behavior of the integral of various kind of sequences of measure-related structures (measurable functions, measurable sets, measures and their combinations) is somewhat conclusive.