Projective cone

In computer science, in particular in the study of approximation algorithms, an L-reduction ("linear reduction") is a transformation of optimization problems which linearly preserves approximability features. L-reductions in studies of approximability of optimization problems play a similar role to that of polynomial reductions in the studies of computational complexity of decision problems.

The term L reduction is sometimes used to refer to log-space reductions, by analogy with the complexity class L, but this is a different concept.

Definition

Let A and B be optimization problems and c_A and c_B their respective cost functions. A pair of functions f and g is an L-reduction if all of the following conditions are met:

• functions f and g are computable in polynomial time,
• if x is an instance of problem A, then f(x) is an instance of problem B,
• if y is a solution to f(x), then g(y) is a solution to x,
• there exists a positive constant α such that

$\mathrm {OPT_{B}} (f(x))\leq \alpha \mathrm {OPT_{A}} (x)$,

• there exists a positive constant β such that for every solution y to f(x)

$|\mathrm {OPT_{A}} (x)-c_{A}(g(y))|\leq \beta |\mathrm {OPT_{B}} (f(x))-c_{B}(y)|$.

Properties

Let a (1±ε)-approximation algorithm f for a problem A be such that $c_{\mathrm {A} }(f(x))$ is at most $\varepsilon \cdot \mathrm {OPT_{A}} (x)$ away from $\mathrm {OPT_{A}} (x)$, for every instance x. (In this notation, + implicitly means a minimization problem, and $-$ means a maximization problem.)

The main point of an L-reduction is the following: given a (f,g,α,β) L-reduction from problem A to problem B, and a (1±ε)-approximation algorithm for B, we obtain a polynomial-time (1±δ)-approximation algorithm for A where $\delta =\alpha \beta \varepsilon$. This implies that if B has a polynomial-time approximation scheme then so does A.

See also

• MAXSNP
• PTAS reduction
• Dominating set#L-reductions: an example with α = β = 1

References

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• Pierluigi Crescenzi: A Short Guide to Approximation Preserving Reductions. In: Proceedings of the Twelfth Annual IEEE Conference on Computational Complexity, June 24-27, 1997, Ulm, Germany. Pages 262-273. IEEE Computer Society Press, 1997. ISBN 0-8186-7907-7
• G. Ausiello, P. Crescenzi, G. Gambosi, V. Kann, A. Marchetti-Spaccamela, M. Protasi. Complexity and Approximation. Combinatorial optimization problems and their approximability properties. 1999, Springer. ISBN 3-540-65431-3