Complexity of finding large grid minors What is the complexity of finding the largest $k\times k$ grid graph that is a minor of a given graph $G$? It is FPT in $k$, and it seems likely to be NP-hard (or NP-complete in a decision version asking whether there exists such a minor for given $k$) but I don't know of a published proof.

There are some papers on constant factor approximations to this problem, e.g.:


Demaine, E.D.; Hajiaghayi, M.; Kawarabayashi, K. (2005). Algorithmic graph minor theory: Decomposition, approximation, and coloring. FOCS, 637–646.
Gua, Q.-P.; Tamaki, H. (2011). Constant-factor approximations of branch-decomposition and largest grid minor of planar graphs in $O(n^{1+\epsilon})$ time. Theoretical Computer Science 412 (32): 4100–4109.


Is it hard to $(1+\epsilon)$-approximate, for some $\epsilon&gt;0$?
If I understood well the problem, perhaps this is an idea for a reduction from the Hamiltonian path problem: given $G$ with $|V| = n$, a source and target node $s, t \in V$;  you can extend it adding a $(n-1) \times n$ "full" grid graph having the bottom-left node of the last row connected to $s$ and the bottom right node of the last row connected to $t$.

Then connect each of the $n-2$ remaining nodes of the last row to each of the nodes in $|V| \setminus \{s,t\}$ adding $(n-2)^2$ edges.

The resulting graph $G'$ has a graph minor of size $n \times n$ if and only if there is an Hamiltonian path from $s$ to $t$ in the source graph $G$.

In the following picture a no instance (left) and a yes instance (right) with the corresponding $n \times n$ grid graph minor.