A Descriptive Definition of a Variational Integral and Applications W. Pfeffer Indiana University Mathematics Journal 1991 text pdf 10.1512/iumj.1991.40.40011 10.1512/iumj.1991.40.40011 en Indiana Univ. Math. J. 40 (1991) 259 - 270 state-of-the-art mathematics http://iumj.org/access/