no. 4
An intermediate existence theory in the calculus of variations
Annali della Scuola Normale Superiore di Pisa - Classe di Scienze, Série 4, Tome 16 (1989) no. 4, pp. 487-526. 
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     title = {An intermediate existence theory in the calculus of variations},
     journal = {Annali della Scuola Normale Superiore di Pisa - Classe di Scienze},
     pages = {487--526},
     publisher = {Scuola normale superiore},
     volume = {Ser. 4, 16},
     number = {4},
     year = {1989},
     mrnumber = {1052732},
     zbl = {0727.49004},
     language = {en},
     url = {http://archive.numdam.org/item/ASNSP_1989_4_16_4_487_0/}
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Clarke, Frank H.; Loewen, Philip D. An intermediate existence theory in the calculus of variations. Annali della Scuola Normale Superiore di Pisa - Classe di Scienze, Série 4, Tome 16 (1989) no. 4, pp. 487-526. http://archive.numdam.org/item/ASNSP_1989_4_16_4_487_0/

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[13] L. Tonelli, Sur une méthode directe du calcul des variations, Rend. Circ. Mat. Palermo 39 (1915), 233-264; also in Opere Scelte (Vol. 2), 289-333, Cremonese, Rome, 1961. | JFM

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