Decomposing a graph into subgraphs with small components.

The component size of a graph is the maximum number of edges in any connected component of the graph. Given a graph $G$ and two integers $k$ and $c$, $(k,c)$-Decomposition is the problem of deciding whether $G$ admits an edge partition into $k$ subgraphs with component size at most $c$. We prove that for any fixed $k \ge 2$ and $c \ge 2$, $(k,c)$-Decomposition is NP-complete in bipartite graphs. Also, when both $k$ and $c$ are part of the input, $(k,c)$-Decomposition is NP-complete even in trees. Moreover, $(k,c)$-Decomposition in trees is W[1]-hard with parameter $k$, and is FPT with parameter $c$. In addition, we present approximation algorithms for decomposing a tree either into the minimum number of subgraphs with component size at most $c$, or into $k$ subgraphs minimizing the maximum component size. En route to these results, we also obtain a fixed-parameter algorithm for Bin Packing with the bin capacity as parameter.