Cuthill–McKee algorithm

Cuthill-McKee ordering of a matrix

RCM ordering of the same matrix

In numerical linear algebra, the Cuthill–McKee algorithm (CM), named for Elizabeth Cuthill and James^[1] McKee,^[2] is an algorithm to permute a sparse matrix that has a symmetric sparsity pattern into a band matrix form with a small bandwidth. The reverse Cuthill–McKee algorithm (RCM) due to Alan George is the same algorithm but with the resulting index numbers reversed.^[3] In practice this generally results in less fill-in than the CM ordering when Gaussian elimination is applied.^[4]

The Cuthill McKee algorithm is a variant of the standard breadth-first search algorithm used in graph algorithms. It starts with a peripheral node and then generates levels $R_{i}$ for $i=1, 2,..$ until all nodes are exhausted. The set $R_{i+1}$ is created from set $R_i$ by listing all vertices adjacent to all nodes in $R_{i}$ . These nodes are listed in increasing degree. This last detail is the only difference with the breadth-first search algorithm.

Algorithm

Given a symmetric $n\times n$ matrix we visualize the matrix as the adjacency matrix of a graph. The Cuthill–McKee algorithm is then a relabeling of the vertices of the graph to reduce the bandwidth of the adjacency matrix.

The algorithm produces an ordered n-tuple $R$ of vertices which is the new order of the vertices.

First we choose a peripheral vertex (the vertex with the lowest degree) $x$ and set $R:=(\{x\})$ .

Then for $i = 1,2,\dots$ we iterate the following steps while $|R|<n$

Construct the adjacency set $A_{i}$ of $R_{i}$ (with $R_{i}$ the i-th component of $R$ ) and exclude the vertices we already have in $R$

A_i := \operatorname{Adj}(R_i) \setminus R

Sort $A_{i}$ with ascending vertex order (vertex degree).
Append $A_{i}$ to the Result set $R$ .

In other words, number the vertices according to a particular breadth-first traversal where neighboring vertices are visited in order from lowest to highest vertex order.

References

↑ Recommendations for ship hull surface representation, page 6
↑ E. Cuthill and J. McKee. Reducing the bandwidth of sparse symmetric matrices In Proc. 24th Nat. Conf. ACM, pages 157–172, 1969.
↑ http://ciprian-zavoianu.blogspot.ch/2009/01/project-bandwidth-reduction.html
↑ J. A. George and J. W-H. Liu, Computer Solution of Large Sparse Positive Definite Systems, Prentice-Hall, 1981

Cuthill–McKee documentation for the Boost C++ Libraries.
A detailed description of the Cuthill–McKee algorithm.
symrcm MATLAB's implementation of RCM.
reverse_cuthill_mckee RCM routine from SciPy written in Cython.

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Cuthill–McKee algorithm

Algorithm

See also

References