The weak saturation number of K_2, t

For two graphs G and F, we say that G is weakly F-saturated if G contains no copy of F as a subgraph and one could join all the nonadjacent pairs of vertices of G in some order so that a new copy of F is created at each step. The weak saturation number wsat(n, F) is the minimum number of edges of a weakly F-saturated graph on n vertices. In this paper, we examine wsat(n, K_s, t), where K_s, t is the complete bipartite graph with parts of sizes s and t. We determine wsat(n, K_2, t), correcting a previous report in the literature. It is also shown that wsat(s+t, K_s,t)=s+t-12 if (s, t)=1 and wsat(s+t, K_s,t)=s+t-12+1, otherwise.