Differentiable Maps with Non-Negative Jacobian P. Church Indiana University Mathematics Journal 1967 text pdf 10.1512/iumj.1967.16.16045 10.1512/iumj.1967.16.16045 en Indiana Univ. Math. J. 16 (1967) 703 - 708 state-of-the-art mathematics http://iumj.org/access/