i think statement should be true because in this question it says **weight of every edge** in graph is increased by 1 . so spanning tree and shortestpath will remain same.

5 votes

Given an undirected weighted graph $G = (V, E)$ with non-negative edge weights, we can compute a minimum cost spanning tree $T = (V, E')$. We can also compute, for a given source vertex $s \epsilon V$ , the shortest paths from s to every other vertex in $V$. We now increase the weight of every edge in the graph by $1$. Are the following true or false, regardless of the structure of $G$? Give a mathematically sound argument if you claim the statement is true or a counterexample if the statement is false.

All the shortest paths from $s$ to the other vertices are unchanged.

All the shortest paths from $s$ to the other vertices are unchanged.

12 votes

Best answer

The given statement "All the shortest paths from s to the other vertices are unchanged." is false . From the above graph it is clear that the shortest path from $S$ to $D$ is $S\implies A\implies B\implies C\implies D$ and the cost is $6$.

Now, we increment the edge cost of all the edges by $1$.

After the increment, the shortest path from S to D gets changed. Now the shortest path becomes $S \implies E \implies D$ and shortest path cost is $9$. The above graph is the proof.