[magnio]

The article you linked argues for a change in the way multiplication is explained to children, not the way it is defined.

> Telling students falsehoods on the assumption that they can be corrected later is rarely a good idea. And telling them that multiplication is repeated addition definitely requires undoing later.

I disagree. Understanding multiplication as repeated addition has always been an invaluable intuition, especially in the beginning, where explicit calculations are important. The biggest hurdle when introducing multiplication is getting them to understand the multiplication table. The fact that it is defined as a separate operation in the definition of ring/field is almost irrelevant in the pedagogical context, just as we don't start teaching real numbers with Dedekind cuts.

[Retric]

The issue with that repeated addition intuition is it keeps breaking down.

How do you define pi * e, with addition? How do you define 2m * 2m, with addition?

Those feel completely intuitive now, but students can get the first far more easily than the second.

[lixtra]

How do you define pi or e but as a limit? At least for practical purposes.

Once you approximate them with rationals you can also imagine adding a fraction of the second multiplicand.

[Retric]

That feels really intuitive now, but less so for a student working their way up to it.

Even more basic is the 2.7 * 3.1 = 27 * 31 and what do I do with the decimal place question. Kids first intuition is often 83.7 because it was one from the right in the numbers they started with.

In that context pi * e exposes several different challenges to peoples mental models of multiplication. Granted most people are just going to plug it into a calculator and trust the answer without much thought, but such is life.

[anon291]

How do you explain pi * e?

It's clear the multiplication as repeated addition holds for the natural numbers, which form a closed ring anyway. Pi and E are sufficiently advanced that by the time you get there you must understand that the operation being described by multiplication is not the same operation at all (depending on how one constructs the real numbers, multiplication of reals is multiplication of sets or functions)