Is the decision whether a $k$-clique exists in a $k$-partite graph NP-hard? I have found only a very limited number of references on this problem, and they seem to be concerned with heuristics to enumerate the cliques (in particular k-cliques in k-partite graphs). On complexity, they only comment that the max-clique problem is generally hard, but nothing on the specific case. Note: This is an edit of my earlier question: whether the max-clique problem in a $k$-partite graph is NP-hard, with $k$ being part of the input? As Austin pointed out in the comments, it is easy to see that the answer is trivially yes by a reduction from the general max-clique problem; any graph $G$ on $n$ vertices can be considered $n$-partite. The new question, however, is more specific and a reduction does not seem so obvious. For example, (and contrary to the original question) for $k=n$ one can easily check if an $n$-partite graph contains/is an $n$-clique. What about general $k$?
$k$-clique in $k$-partite graph
np hardness;clique
This should be indeed NP-hard. And the construction is very similar to one that already worked for a similar question: Many-one reduction from inequality problem to equality problem.From a graph $G=(V,E)$ and $k$ construct the graph $G'$ with vertex set $V \times \{1,\dots,k\}$ and edges between $(v,i)$ and $(w,j)$ if $(i \ne j)$ and $(v \ne w)$ and $(v,w) \in E$. It is easy to see that $G'$ is $k$-partite and it has a $k$-clique if and only if $G$ has.