Ask HN: Threshold Concepts in Mathematics

2 days ago 4

In mathematics education we can find mention of *threshold concepts*. These are mathematical concepts that:

1) are notoriously difficult to grasp, both intuitively, conceptually and computationally; 2) are central and critical for furthering one's mathematical understanding; 3) eventually become quite easy ("How is it I could not understand it before?"), because we employ them so much that any mathematician will internalize them after a while.

Setting bar for these three criteria high, four threshold concepts come to my mind:

- basic algebra (it is well-known that many children struggle a lot with middle school maths when transitioning for arithmetics to algebra); - differentiation and integration (AFAIK, differentiation seems more difficult of these two for most students, because it makes them think about graphs in a novel way); - delta-epsilon arguments in real analysis (as 99% undergraduate students in maths can confirm :)); - forcing in advanced set theory (I know nothing about it myself, but I have read several places that it can be a backbreaker; but I am not sure whether it satisfies (3)).

Do you know other examples of such threshold concepts in mathematics?

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