Abstract
The multiplication principle (MP) is a fundamental aspect of combinatorial enumeration. In an effort to better understand students’ reasoning about the MP, we had two undergraduate students reinvent a statement of the MP in a teaching experiment. In this paper, we adopt an actor-oriented perspective (Lobato, Educational Researcher, 32(1), 17–20, 2003, Educational Psychologist, 47(3), 1–16 2012) and draw on Lockwood’s, Journal of Mathematical Behavior, 32, 251–265 (2013) model of students’ combinatorial thinking to trace the students’ development of a certain conception about order – that a statement of the MP should allow for any ordering of the stages of a counting process. Notably, this conception differs from how order is treated in textbook statements of the MP. We report on how this conception persisted for the students, and we explain how the researchers (interviewers) and students managed to proceed through the teaching experiment while maintaining different conceptions of order. The students’ reasoning about order sheds light on ways in which students may think about order and highlights potential subtleties about multiplication in counting that deserve attention.
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Notes
We do not mean to infer too much from how an expression is written in terms of actually reflecting one’s counting process. For example, someone could actually engage in the process of first considering the 5 options of the last position (so, effectively going through the process of 5⋅9⋅8⋅7⋅6) but write it as 9⋅8⋅7⋅6⋅5, and so the order in which they actually performed the stages is lost in the written inscription. Here we write 5⋅9⋅8⋅7⋅6 intentionally to describe the order of the stages, where the first stage is written first (or leftmost) and the last stage is written last (or rightmost). This is to facilitate our own communication of the stages, but we acknowledge that we cannot infer an order of stages just from a written expression without having confirmation of what the writer of that expression intends.
Hour-long interviews occurred on Wednesday and Friday of Week 1, Monday and Wednesday of Week 2, Friday of Week 3, and Monday, Wednesday, and Friday of Week 4.
Another solution is to say there are 64 options for the first rook and then 14 remaining places for the second rook, yielding an expression of 64⋅14.
Said another way, this is a Cartesian Product problem that does not specify the order in which the elements of the k-tuple are generated or listed. That is, a (coin, card, dice) 3-tuple is an acceptable outcome, as is a (card, dice, coin) 3-tuple. Such problems involve creating k-tuples out of k predetermined, independent sets.
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Lockwood, E., Purdy, B. An Unexpected Outcome: Students’ Focus on Order in the Multiplication Principle. Int. J. Res. Undergrad. Math. Ed. 6, 213–244 (2020). https://doi.org/10.1007/s40753-019-00107-3
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DOI: https://doi.org/10.1007/s40753-019-00107-3