The definition of graph isomorphism was added. Information about graph K2 walk was added.

[dimbo4ka]

No description provided.

[Lipen]

Thanks for your contribution!

Note however that it is better to dedicate each PR to a single feature or change. For example, it would be better to devote this PR to isomorphism definition only, and submit another PR with the note about the actual possibility of a closed walk with distinct vertices, but repeated edges in the very specific case.

Regarding the latter, it is not exactly about K_2, but it is about the walk being "K2-like", i.e. "v-e-u-e-v", which is possible not only in K_2, but even in larger graphs, since we do not have to visit all vertices. So, this note should be explained separately, better with a visualization. I would also suggest keeping the "(impossible)" in the last row, and just adding a star (impossible*) with the aforementioned clarification (\footnote{Impossible in general case, the only exception is the v-e-u-e-v closed walk.}).

Let's split this PR into two, rename this one, add some non-empty description (even I do this, see my PRs at work, so you can definitely come up with some generic PR description!), and merge, since the part about the isomorphism looks good. It is also necessary to double check that the tight layout does not break after the added text, but I will do it myself.

[dimbo4ka]

Thanks so much for the feedback! I'll fix the flaws.