COMPUTING SCIENCE

# Accidental Algorithms

A strange new family of algorithms probes the boundary between easy and hard problems

# The Match Game

To understand the new holographic algorithms, we need one more ingredient from graph theory: the idea of a perfect matching.

Consider the double-feature festival. You want to show movies in pairs, with the proviso that any two films scheduled together should have a performer in common; also, no film can be screened more than once. These constraints lead to a graph where the vertices are film titles, and two titles are connected by an edge if the films share an actor. The task is to identify a set of edges linking each vertex to exactly one other vertex. The brute-force method of trying all possible matchings is exponential, but if you are given a candidate solution, you can efficiently verify its correctness:

Thus the perfect-matching problem lies in NP.

In the 1960s Jack Edmonds, now of the University of Waterloo, devised an efficient algorithm that finds a perfect matching if there is one. The Edmonds algorithm works in polynomial time, which means the decision problem for perfect matching is in P. (Indeed, Edmonds's 1965 paper includes the first published discussion of the distinction between polynomial and exponential algorithms.)

Another success story among matching methods applies only to planar graphs—those that can be drawn without crossed edges. On a planar graph you can efficiently solve not only the decision problem for perfect matching but also the counting problem—that is, you can learn how many different subsets of edges yield a perfect matching. In general, counting problems seem more difficult than decision problems, since the solution conveys more information. The main complexity class for counting problems is called #P (pronounced "sharp P"); it includes NP as a subset, so #P problems must be at least as hard as NP.

The problem of counting planar perfect matchings has its roots in physics and chemistry, where the original question was: If diatomic molecules are adsorbed on a surface, forming a single layer, how many ways can they be arranged? Another version asks how many ways dominos (2-by-1 rectangles) can be placed on a chessboard without gaps or overlaps. The answers exhibit clear signs of exponential growth; when you arrange dominos on square boards of size 2, 4, 6 and 8, the number of distinct tilings is 2, 36, 6,728 and 12,988,816. Given this rapid proliferation, it seems quite remarkable that a polynomial-time algorithm can count the configurations. The ingenious method was developed in the early 1960s by Pieter W. Kasteleyn and, independently, Michael E. Fisher and H. N. V. Temperley. It has come to be known as the FKT algorithm.

The mathematics behind the FKT algorithm takes some explaining. In outline, the idea is to encode the structure of an *n*-vertex graph in an *n*-by-*n* matrix; then the number of perfect matchings is given by an easily computed property of the matrix. The illustration *(above right)* shows how the graph is represented in matrix form.

The computation performed on the matrix is essentially the evaluation of a determinant. By definition, a determinant is a sum of *n*! terms, where each term is a product of *n* elements chosen from the matrix. The symbol *n*! denotes the factorial of *n*, or in other words *n*×(*n*–1)×...×3×2×1. The trouble is, *n*! is not a polynomial function of *n*; it qualifies as an exponential. Thus, under the rules of complexity theory, the whole scheme is really no better than the brute-force enumeration of all perfect matchings. But this is where the rabbit comes out of the hat. There are alternative algorithms for computing determinants that *do* achieve polynomial performance; the best-known example is the technique called Gaussian elimination. With these methods, all but a polynomial number of terms in that giant summation magically cancel out. We never have to compute them, or even look at them.

(The answer sought in the perfect-matching problem is actually not the determinant but a related quantity called the Pfaffian. However, the Pfaffian is equal to the square root of the determinant, and so the computational procedure is essentially the same.)

The existence of a shortcut for evaluating determinants and Pfaffians is like a loophole in the tax code—a windfall for those who can take advantage of it, but you can only get away with such special privileges if you meet very stringent conditions.

Closely related to the determinant is another quantity associated with matrices called the permanent. It's another sum of *n*! products, but even simpler. For the determinant, a complicated rule assigns positive and negative signs to the various terms of the summation. For the permanent, there's no need to bother keeping track of the signs; they're all positive. But the alternation of signs is necessary for the cancellations that allow fast computation of determinants. As a result, the polynomial loophole doesn't work for permanents. In 1979 Valiant showed that the calculation of permanents is #P-complete. (It was in this work that the class #P was first defined.)

At a higher level, too, the conspiracy of circumstances that allows perfect matchings to be counted in polynomial time seems rather delicate and sensitive to details. The algorithm works only for planar graphs; attempts to extend it to larger families of graphs have failed. Even for planar graphs, it works only for *perfect* matchings; counting the total number of matchings is a #P-complete task.

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