@article{RSMUP_1974__52__1_0,
     author = {Appling, William D. L.},
     title = {Upper and lower integral difference functionals, closest approximations, and integrability},
     journal = {Rendiconti del Seminario Matematico della Universit\`a di Padova},
     pages = {1--24},
     publisher = {Seminario Matematico of the University of Padua},
     volume = {52},
     year = {1974},
     mrnumber = {404565},
     zbl = {0325.28006},
     language = {en},
     url = {http://archive.numdam.org/item/RSMUP_1974__52__1_0/}
}