What do you mean by important breakthroughs? For understanding my question see these levels in math research: ordinary paper < good quality paper < systematic papers answering some important conjectures < works that lead to a new sub-branch of math (like Ricci flow inside of differential geometry/topology) < works that discover/introduce a new branch of math (such k-theory and category theory) and so on. There is also a breakthrough prize in math. Which one is related to your post?

The Milnor book in 1963 is another example. I wonder how they write sub(p)script with typewriter? how they change the font size or math/Greek special symbols?

The added info is so interesting. Now I understand why he is very thankful to Chern.

@CarloBeenakker: Do you know the full name of Robert?

There is a proof of Total coloring conjecture on arxiv. posted on 2020 but I am not aware of its correctness.

Do you aware of this recent proof of Total coloring conjecture whether it is correct/accepted or not?

I am curious why this question is interesting? what is its application?