Graph Has Two / Three Different Minimal Spanning Trees ? - algorithm

I'm trying to find an efficient method of detecting whether a given graph G has two different minimal spanning trees. I'm also trying to find a method to check whether it has 3 different minimal spanning trees. The naive solution that I've though about is running Kruskal's algorithm once and finding the total weight of the minimal spanning tree. Later , removing an edge from the graph and running Kruskal's algorithm again and checking if the weight of the new tree is the weight of the original minimal spanning tree , and so for each edge in the graph. The runtime is O(|V||E|log|V|) which is not good at all, and I think there's a better way to do it.
Any suggestion would be helpful,
thanks in advance

You can modify Kruskal's algorithm to do this.
First, sort the edges by weight. Then, for each weight in ascending order, filter out all irrelevant edges. The relevant edges form a graph on the connected components of the minimum-spanning-forest-so-far. You can count the number of spanning trees in this graph. Take the product over all weights and you've counted the total number of minimum spanning trees in the graph.
You recover the same running time as Kruskal's algorithm if you only care about the one-tree, two-trees, and three-or-more-trees cases. I think you wind up doing a determinant calculation or something to enumerate spanning trees in general, so you likely wind up with an O(MM(n)) worst-case in general.

Suppose you have a MST T0 of a graph. Now, if we can get another MST T1, it must have at least one edge E different from the original MST. Throw away E from T1, now the graph is separated into two components. However, in T0, these two components must be connected, so there will be another edge across this two components that has exactly the same weight as E (or we could substitute the one with more weight with the other one and get a smaller ST). This means substitute this other edge with E will give you another MST.
What this implies is if there are more than one MSTs, we can always change just a single edge from a MST and get another MST. So if you are checking for each edge, try to substitute the edge with the ones with the same weight and if you get another ST it is a MST, you will get a faster algorithm.

Suppose G is a graph with n vertices and m edges; that the weight of any edge e is W(e); and that P is a minimal-weight spanning tree on G, weighing Cost(W,P).
Let δ = minimal positive difference between any two edge weights. (If all the edge weights are the same, then δ is indeterminate; but in this case, any ST is an MST so it doesn't matter.) Take ε such that δ > n·ε > 0.
Create a new weight function U() with U(e)=W(e)+ε when e is in P, else U(e)=W(e). Compute Q, an MST of G under U. If Cost(U,Q) < Cost(U,P) then Q≠P. But Cost(W,Q) = Cost(W,P) by construction of δ and ε. Hence P and Q are distinct MSTs of G under W. If Cost(U,Q) ≥ Cost(U,P) then Q=P and distinct MSTs of G under W do not exist.
The method above determines if there are at least two distinct MSTs, in time O(h(n,m)) if O(h(n,m)) bounds the time to find an MST of G.
I don't know if a similar method can treat whether three (or more) distinct MSTs exist; simple extensions of it fall to simple counterexamples.

Related

Minimum Spanning Tree with only 2 equal edges

Assume you have an undirected-weighted graph G, with different edges weighs but for only two edges: w(e1)=w(e2)
I have to prove that G has at most one minimum spanning tree which includes e1.
Also I have to prove that G has at most one minimum spanning tree which doesnt include e1.
I only need a solution for the first one and will solve the second one alone.
Thanks
For solving part 1:
Consider the graph you get by removing e1 from G (and possibly one of it's vertices, if it's now not connected to the rest of the graph), let's call it G'.
In this graph (G') , all the edge weights are different.
Now suppose G has more than 1 MST which includes e1 - they would both be different MSTs for G'.
Now the trick is that there's a theorem that in this kind of graph (all edges are different), the MST is unique. see the proof(s) here.
edit: You can probably just take the proof from the link and edit it slightly for your case.

Finding MST such that a specific vertex has a minimum degree

Given undirected, connected graph G={V,E}, a vertex in V(G), label him v, and a weight function f:E->R+(Positive real numbers), I need to find a MST such that v's degree is minimal. I've already noticed that if all the edges has unique weight, the MST is unique, so I believe it has something to do with repetitive weights on edges. I though about running Kruskal's algorithm, but when sorting the edges, I'll always consider edges that occur on v last. For example, if (a,b),(c,d),(v,e) are the only edges of weight k, so the possible permutations of these edges in the sorted edges array are: {(a,b),(c,d),(v,e)} or {(c,d),(a,b),(v,e)}. I've ran this variation over several graphs and it seems to work, but I couldn't prove it. Does anyone know how to prove the algorithm's correct (Meaning proving v's degree is minimal), or give a contrary example of the algorithm failing?
First note that Kruskal's algorithm can be applied to any weighted graph, whether or not it is connected. In general it results in a minimum-weight spanning forest (MSF), with one MST for each connected component. To prove that your modification of Kruskal's algorithm succeeds in finding the MST for which v has minimal degree, it helps to prove the slightly stronger result that if you apply your algorithm to a possibly disconnected graph then it succeeds in finding the MSF where the degree of v is minimized.
The proof is by induction on the number, k, of distinct weights.
Basis Case (k = 1). In this case weights can be ignored and we are trying to find a spanning forest in which the degree of v is minimized. In this case, your algorithm can be described as follows: pick edges for as long as possible according to the following two rules:
1) No selected edge forms a cycle with previously selected edges
2) An edge involving v isn't selected unless any edge which doesn't
involve v violates rule 1.
Let G' denote the graph from which v and all incident edges have been removed from G. It is easy to see that the algorithm in this special case works as follows. It starts by creating a spanning forest for G'. Then it takes those trees in the forest that are contained in v's connected component in the original graph G and connects each component to v by a single edge. Since the components connected to v in the second stage can be connected to each other in no other way (since if any connecting edge not involving v exists it would have been selected by rule 2) it is easy to see that the degree of v is minimal.
Inductive Case: Suppose that the result is true for k and G is a weighted graph with k+1 distinct weights and v is a specified vertex in G. Sort the distinct weights in increasing order (so that weight k+1 is the longest of the distinct weights -- say w_{k+1}). Let G' be the sub-graph of G with the same vertex set but with all edges of weight w_{k+1} removed. Since the edges are sorted in the order of increasing weight, note that the modified Kruskal's algorithm in effect starts by applying itself to G'. Thus -- by the induction hypothesis prior to considering edges of weight w_{k+1}, the algorithm has succeeded in constructing an MSF F' of G' for which the degree, d' of v in G' is minimized.
As a final step, modified Kruskal's applied to the overall graph G will merge certain of the trees in F' together by adding edges of weight w_{k+1}. One way to conceptualize the final step is the think of F' as a graph where two trees are connected exactly when there is an edge of weight w_{k+1} from some node in the first tree to some node in the second tree. We have (almost) the basis case with F'. Modified Kruskal's will add edged of weight w_{k+1} until it can't do so anymore -- and won't add an edge connecting to v unless there is no other way to connect to trees in F' that need to be connected to get a spanning forest for the original graph G.
The final degree of v in the resulting MSF is d = d'+d" where d" is the number of edges of weight w_{k+1} added at the final step. Neither d' nor d" can be made any smaller, hence it follows that d can't be made any smaller (since the degree of v in any spanning forest can be written as the sum of the number of edges whose weight is less than w_{k+1} coming into v and the number off edges of weight w_{k+1} coming into v).
QED.
There is still an element of hand-waving in this, especially with the final step -- but Stack Overflow isn't a peer-reviewed journal. Anyway, the overall logic should be clear enough.
One final remark -- it seems fairly clear that Prim's algorithm can be similarly modified for this problem. Have you looked into that?

Prim and Kruskal's algorithms complexity

Given an undirected connected graph with weights. w:E->{1,2,3,4,5,6,7} - meaning there is only 7 weights possible.
I need to find a spanning tree using Prim's algorithm in O(n+m) and Kruskal's algorithm in O( m*a(m,n)).
I have no idea how to do this and really need some guidance about how the weights can help me in here.
You can sort edges weights faster.
In Kruskal algorithm you don't need O(M lg M) sort, you just can use count sort (or any other O(M) algorithm). So the final complexity is then O(M) for sorting and O(Ma(m)) for union-find phase. In total it is O(Ma(m)).
For the case of Prim algorithm. You don't need to use heap, you need 7 lists/queues/arrays/anything (with constant time insert and retrieval), one for each weight. And then when you are looking for cheapest outgoing edge you check is one of these lists is nonempty (from the cheapest one) and use that edge. Since 7 is a constant, whole algorithms runs in O(M) time.
As I understand, it is not popular to answer homework assignments, but this could hopefully be usefull for other people than just you ;)
Prim:
Prim is an algorithm for finding a minimum spanning tree (MST), just as Kruskal is.
An easy way to visualize the algorithm, is to draw the graph out on a piece of paper.
Then you create a moveable line (cut) over all the nodes you have selected. In the example below, the set A will be the nodes inside the cut. Then you chose the smallest edge running through the cut, i.e. from a node inside of the line to a node on the outside. Always chose the edge with the lowest weight. After adding the new node, you move the cut, so it contains the newly added node. Then you repeat untill all nodes are within the cut.
A short summary of the algorithm is:
Create a set, A, which will contain the chosen verticies. It will initially contain a random starting node, chosen by you.
Create another set, B. This will initially be empty and used to mark all chosen edges.
Choose an edge E (u, v), that is, an edge from node u to node v. The edge E must be the edge with the smallest weight, which has node u within the set A and v is not inside A. (If there are several edges with equal weight, any can be chosen at random)
Add the edge (u, v) to the set B and v to the set A.
Repeat step 3 and 4 until A = V, where V is the set of all verticies.
The set A and B now describe you spanning tree! The MST will contain the nodes within A and B will describe how they connect.
Kruskal:
Kruskal is similar to Prim, except you have no cut. So you always chose the smallest edge.
Create a set A, which initially is empty. It will be used to store chosen edges.
Chose the edge E with minimum weight from the set E, which is not already in A. (u,v) = (v,u), so you can only traverse the edge one direction.
Add E to A.
Repeat 2 and 3 untill A and E are equal, that is, untill you have chosen all edges.
I am unsure about the exact performance on these algorithms, but I assume Kruskal is O(E log E) and the performance of Prim is based on which data structure you use to store the edges. If you use a binary heap, searching for the smallest edge is faster than if you use an adjacency matrix for storing the minimum edge.
Hope this helps!

minimum connected subgraph containing a given set of nodes

I have an unweighted, connected graph. I want to find a connected subgraph that definitely includes a certain set of nodes, and as few extras as possible. How could this be accomplished?
Just in case, I'll restate the question using more precise language. Let G(V,E) be an unweighted, undirected, connected graph. Let N be some subset of V. What's the best way to find the smallest connected subgraph G'(V',E') of G(V,E) such that N is a subset of V'?
Approximations are fine.
This is exactly the well-known NP-hard Steiner Tree problem. Without more details on what your instances look like, it's hard to give advice on an appropriate algorithm.
I can't think of an efficient algorithm to find the optimal solution, but assuming that your input graph is dense, the following might work well enough:
Convert your input graph G(V, E) to a weighted graph G'(N, D), where N is the subset of vertices you want to cover and D is distances (path lengths) between corresponding vertices in the original graph. This will "collapse" all vertices you don't need into edges.
Compute the minimum spanning tree for G'.
"Expand" the minimum spanning tree by the following procedure: for every edge d in the minimum spanning tree, take the corresponding path in graph G and add all vertices (including endpoints) on the path to the result set V' and all edges in the path to the result set E'.
This algorithm is easy to trip up to give suboptimal solutions. Example case: equilateral triangle where there are vertices at the corners, in midpoints of sides and in the middle of the triangle, and edges along the sides and from the corners to the middle of the triangle. To cover the corners it's enough to pick the single middle point of the triangle, but this algorithm might choose the sides. Nonetheless, if the graph is dense, it should work OK.
The easiest solutions will be the following:
a) based on mst:
- initially, all nodes of V are in V'
- build a minimum spanning tree of the graph G(V,E) - call it T.
- loop: for every leaf v in T that is not in N, delete v from V'.
- repeat loop until all leaves in T are in N.
b) another solution is the following - based on shortest paths tree.
- pick any node in N, call it v, let v be a root of a tree T = {v}.
- remove v from N.
loop:
1) select the shortest path from any node in T and any node in N. the shortest path p: {v, ... , u} where v is in T and u is in N.
2) every node in p is added to V'.
3) every node in p and in N is deleted from N.
--- repeat loop until N is empty.
At the beginning of the algorithm: compute all shortest paths in G using any known efficient algorithm.
Personally, I used this algorithm in one of my papers, but it is more suitable for distributed enviroments.
Let N be the set of nodes that we need to interconnect. We want to build a minimum connected dominating set of the graph G, and we want to give priority for nodes in N.
We give each node u a unique identifier id(u). We let w(u) = 0 if u is in N, otherwise w(1).
We create pair (w(u), id(u)) for each node u.
each node u builds a multiset relay node. That is, a set M(u) of 1-hop neigbhors such that each 2-hop neighbor is a neighbor to at least one node in M(u). [the minimum M(u), the better is the solution].
u is in V' if and only if:
u has the smallest pair (w(u), id(u)) among all its neighbors.
or u is selected in the M(v), where v is a 1-hop neighbor of u with the smallest (w(u),id(u)).
-- the trick when you execute this algorithm in a centralized manner is to be efficient in computing 2-hop neighbors. The best I could get from O(n^3) is to O(n^2.37) by matrix multiplication.
-- I really wish to know what is the approximation ration of this last solution.
I like this reference for heuristics of steiner tree:
The Steiner tree problem, Hwang Frank ; Richards Dana 1955- Winter Pawel 1952
You could try to do the following:
Creating a minimal vertex-cover for the desired nodes N.
Collapse these, possibly unconnected, sub-graphs into "large" nodes. That is, for each sub-graph, remove it from the graph, and replace it with a new node. Call this set of nodes N'.
Do a minimal vertex-cover of the nodes in N'.
"Unpack" the nodes in N'.
Not sure whether or not it gives you an approximation within some specific bound or so. You could perhaps even trick the algorithm to make some really stupid decisions.
As already pointed out, this is the Steiner tree problem in graphs. However, an important detail is that all edges should have weight 1. Because |V'| = |E'| + 1 for any Steiner tree (V',E'), this achieves exactly what you want.
For solving it, I would suggest the following Steiner tree solver (to be transparent: I am one of the developers):
https://scipjack.zib.de/
For graphs with a few thousand edges, you will usually get an optimal solution in less than 0.1 seconds.

Calculating total number of spanning trees containing a particular set of edges

I have tried the following approach:
First I do edge contraction for all the edges in the given set of edges to form a modified graph.
Then I calculate the total number of spanning trees, using the matrix tree theorem, from the modified graph.
I want to know if this method is correct and if there are some other better methods.
Let G be a graph, let e be an edge, and let G/e be the same graph with e contracted. Then,
Proposition: There is a bijection between the spanning trees of G that contain e, and the spanning trees of G/e.
This proposition is not hard to prove; you're better off understanding the proof yourself instead of just asking other people whether it's true. Obviously if you have a spanning T tree of G that contains e, then T/e is a spanning tree of G/e. The thing to think through is that you can also go backwards.
And, as Adam points out, you have to be careful to properly handle graphs with parallel edges and graphs with edges from a vertex to itself.
I don't know if it's correct or not, but you'll have to be careful of the fact that edge contraction can lead to parallel edges. You'll have to make sure that trees differing only by which parallel edge is used are counted as being distinct.

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