On Reciprocal Variational Principles for Perfect Fluids M. Sewell Indiana University Mathematics Journal 1963 text pdf 10.1512/iumj.1963.12.12031 10.1512/iumj.1963.12.12031 en Indiana Univ. Math. J. 12 (1963) 495 - 504 state-of-the-art mathematics http://iumj.org/access/