How do you prove a graph **is not** planar?
38473
Checking whether a graph is planar
126192
Connected, locally connected, path-connected but not locally path-connected subspace of the plane
116932
Proving a simple graph is a connected graph
57867
"Planar" graphs on Möbius strips
150707
Prove the statement 'World is not flat'
117685
Prove that a bipartite planar graph has a vertex of degree at most 3
32917
Show that, for $t>0$, $\log t$ is not a polynomial.
126428
Prove the edges of a multigraph may be oriented such that the net-degree of any vertex is $\leq 1$.
27407
every planar graph can be expressed as a union of five edge-disjoint forests
145181
How can I prove the maximum number of edges?
129288
proof that topologist sine curve is not locally connected
145718
Proving that A connected graph on $n$ vertices is a tree iff it has $n-1$ edges
42714
A diagram which is not the torus
11329
Proving a graph has no Hamiltonian cycle
35115
Questions on how to prove that a set of connectives is NOT functionally complete
25874
Showing that a certain map is not flat by explicit counterexample
54875
How to prove polynomials with degree $n$ does not form a vector space?