K-Clustering of a Graph maximizing intra-cluster weights? I would like to know if the following problem has already been studied, and if so how is it called. In particular I'm interested in approximability results.

Input: A complete graph G with non-negative integer weights on edges and an integer $K\ge 2$.

Output: A $K$-partition $P=\{P_1, ..., P_K\}$ of $V(G)$

Measure (to maximize): The sum of the weights for the edges with both endpoints in the same set of $P$, i.e.:
$$
M(P)=\sum_{i=1}^k \sum_{u,v\in P_i} w\left( u,v \right)
$$
where $w$ is the edge-weight function.

Thanks in advance for any suggestion.
This problem is called MIN-SUM clustering and is NP-hard. There's a paper by Bartal, Charikar and Raz from 2001 that has an approximation scheme for it: the paper also includes references to the NP-hardness result and other related results.