based in/on Hi, everyone In a paper, I found this: "Recently studies have been done about topologic properties of null electromagnetic fields, such as W. Irvine in 2008 and H. Kedia 2013. Also, Adamo and Newman, in 2009, used the congruence of null geodesic shear-free to study and understand problems related to General Relativity. These studies are BASED IN Robinson’s Theorem, which states that each congruence of null geodesic shear-free is related to a family of null electromagnetic fields and, conversely, each null electromagnetic field is related to a congruence of null geodesic shear-free, and Kerr’s Theorem, which relates a congruence of null geodesic shear-free to the zeros of an arbitrary analytic function." Shouldn't it be "based on"? http://repositorio.unb.br/handle/10482/17155 Thank you