≅ (U+2245), another combination of "≈" and "=", which is used to indicate isomorphism or sometimes congruence
Back-and-forth method, a method of showing isomorphism between countably infinite structures satisfying specified conditions
He introduced the Duflo isomorphism, an isomorphism between the center of the enveloping algebra of a finite-dimensional Lie algebra and the invariants of its symmetric algebra.
Anatoly Maltsev proved that solvable subgroups of the integer general linear group are polycyclic; and later Louis Auslander (1967) and Swan proved the converse, that any polycyclic group is up to isomorphism a group of integer matrices.
Existence of a partial isomorphism entails elementary equivalence, however the converse is not generally true, but it holds for ω-saturated models.
Powell is widely known for his contributions to institutional analysis, beginning with his article with Paul DiMaggio, "The Iron Cage Revisited: Institutional Isomorphism and Collective Rationality in Organizational Fields" (1983) and their subsequent edited book, The New Institutionalism in Organizational Analysis (1991).