Defining and demonstrating an equivalence way of thinking in enumerative combinatorics.

• Equivalence is a fundamental idea within combinatorics (and mathematics more broadly). • We draw on Harel's duality principle and describe and demonstrate an equivalence way of thinking in combinatorics. • This equivalence way of thinking can be valuable for students in a variety of combinatorial situations. Counting problems offer opportunities for rich mathematical thinking, and yet there is evidence that students struggle to solve counting problems correctly. There is a need to identify useful approaches and thought processes that can help students be successful in their combinatorial activity. In this paper, we propose a characterization of an equivalence way of thinking, we discuss examples of how it arises mathematically in a variety of combinatorial concepts, and we offer episodes from a paired teaching experiment with undergraduate students that demonstrate useful ways in which students developed and leverage this way of thinking. Ultimately, we argue that this way of thinking can apply to a variety of combinatorial situations, and we make the case that it is a valuable way of thinking that should be prioritized for students learning combinatorics. [ABSTRACT FROM AUTHOR]