On a classical problem of the calculus of variations without convexity assumptions
Annales de l'I.H.P. Analyse non linéaire, Tome 7 (1990) no. 2, pp. 97-106. 
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     title = {On a classical problem of the calculus of variations without convexity assumptions},
     journal = {Annales de l'I.H.P. Analyse non lin\'eaire},
     pages = {97--106},
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     volume = {7},
     number = {2},
     year = {1990},
     mrnumber = {1051230},
     zbl = {0702.49002},
     language = {en},
     url = {http://archive.numdam.org/item/AIHPC_1990__7_2_97_0/}
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Cellina, A.; Colombo, G. On a classical problem of the calculus of variations without convexity assumptions. Annales de l'I.H.P. Analyse non linéaire, Tome 7 (1990) no. 2, pp. 97-106. http://archive.numdam.org/item/AIHPC_1990__7_2_97_0/

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