ALG(G,H): Let G(0,1) and H(0,1) be the usual adacency matrix using 0 for EDGE NOT THERE and 1 or EDGE IS THERE. Choose n random pairs of numbers (a1,b1), (a2,b2),...,(an,bn) all \le n^2 (happy to change the bound if it will make it work better). For all i see if DET(G(ai,bi))=DET(H(ai,bi)). If there is an i such that they are NOT equal than output NOT ISOM (with complete certainly). If they are ALWAYS equal then output GEE, THOSE GRAPHS ARE PROB ISOMORPHIC.

- I person who used to work on GI told me that he THINKS there are graphs G, H where the polynomials G(x,y) and H(x,y) are identical yet G and H are not isom. So their are some G,H where the above will definitely fail. Not surprising.
- Are such graphs rare? Will this work on `almost all pairs of graphs' ? Not clear what that means.
- If we also checked degree sequences and degrees-of-degrees, would that help? I doubt it since I think that graphs for which this fails are very similar in other ways, but I don't know that.

I think its a good idea and it might work for our purposes (which may be a topic of a later blog).

Pretty sure we tried this years ago and it fails.

ReplyDeletedick lipton

It works poorly if the graph contains twin vertices, for one...

ReplyDeleteHowever, I have no idea about the general power of the scheme.

Frank Harary, The determinant of the adjacency matrix of a graph, SIAM Review, Vol. 4, No. 3. (Jul., 1962), pp. 202-210, It mentions counterexamples with 16 vertices by Bose and others but does not include them, saying that they will be "soon appear in the literature". These are likely strongly regular graphs of some sort. The natural reference is:

ReplyDeleteBose, R.C.: Strongly regular graphs, partial geometries and

partially balanced designs. Pacific J. Math., 13 (1963),

but it is not online. In a later paper, Bose discusses a couple of non-isomorphic graphs on 16 vertices that seem to be the likely candidates but I haven't checked them:

(a) Arrange the integers 1 through 16 corresponding to

the vertices of the graph in a 4 x 4 scheme.

1 2 3 4

5 6 7 8

9 10 11 12

13 14 15 16

Then any two vertices are adjacent if and only if the corresponding

integers are in the same row or same column.

(b) Arrange the integers corresponding to the vertices in a

4 x 4 scheme, and impose on it a cyclic Latin square. We thus obtain

1A 2B 3C 4D

5D 6A 7B 8C

9C 10D llA 12B

13B 14C 15D 16A

Two vertices are adjacent if they do not occur in the same row or

in the same column, and do not come together with the same letter

of the cyclic square.

Threadjack. Do papers on unsuccessful approaches and attempts get published anymore? They seem useful.

ReplyDelete1. The graph pair in page 3 of https://orion.math.iastate.edu/butler/PDF/spectra_lecture_1.pdf is a counter example. The determinant in both the cases are zero.

ReplyDelete2. GI is easy for most graphs. I would guess that this type of graphs of are also rare.

More generally, you can compute and compare the characteristic polynomials [1] of the two input graphs. However, this graph polynomial is not complete, which means that there are non-isomorphic graphs with the same characteristic polynomial [1].

ReplyDelete[1] https://en.wikipedia.org/wiki/Characteristic_polynomial

I'll admit to not entirely following your notation, Bill, but i'll take the opportunity to point out this paper by Steve Butler which gives methods to generate pairs of non-isomorphic graphs which share the same eigenvalues.

ReplyDeleteA construction of cospectral graphs for the normalized Laplacian

The paper's results are focused on the normalized Laplacian matrix, but it includes techniques for several common graph matrices