Showing $| mathbb{N} | =|mathbb{N} times mathbb{N}|$ using the diagonal argument is seeming a little hard for me to prove by an explicit function.

It’s clear that we need a function $f: mathbb{N} to mathbb{N} times mathbb{N}$, and arranging them diagonally seems to work very well.

In other words:

$(0,0) (0,1) (0,2) (0,3) …$

$(1,0) (1,1) (1,2) (1,3) …$

$(2,0) (2,1) (2,2) (2,3) …$

… (Not sure how to put in a table).

And we can say $f(0) = (0,0), f(1) = (0,1), f(2) = (1,0), f(3) = (0,2), f(4) = (1,1) …$, going diagonally all the way through. I’m trying to find an explicit function for $f$ though. The most I’ve come up with is that:

$0$ maps to $(0,0)$

$1$ and $2$ get mapped to the points $(a,b)$ with $a,b in { 0,1 }$, and $a + b = 1$.

$3,4,$ and $5$ get mapped to points $(a,b)$ with $a,b in { 0,1,2 }$ and $a+b = 2$.

$6, 7, 8$, and $9$ get mapped to points $(a,b)$ with $a,b in { 0,1,2,3 }$ and $a + b = 3$.

How can we write an explicit formula for $f(n)?$

I’m aware there are many other ways to show these sets are the same, but I’d like to stick with this technique for now.

Thanks.