The Gap Between Greedy Algorithm and Minimum Multiplicative Spanner

TL;DR

This paper analyzes how closely the greedy algorithm approximates the smallest possible k-spanner in graphs, providing bounds on its universal optimality and introducing a new approximate optimality concept.

Contribution

It offers a detailed, fine-grained analysis of the greedy algorithm's performance relative to universal optimality for various k values, including new bounds and an approximate optimality notion.

Findings

Greedy algorithm is not universally optimal for k< n/3.

Greedy algorithm is universally optimal for k> 2n/3.

Introduces an approximate universal optimality concept with bounds for different k ranges.

Abstract

The greedy algorithm adapted from Kruskal's algorithm is an efficient and folklore way to produce a $k$-spanner with girth at least $k+2$. The greedy algorithm has shown to be `existentially optimal', while it's not `universally optimal' for any constant $k$. Here, `universal optimality' means an algorithm can produce the smallest $k$-spanner $H$ given any $n$-vertex input graph $G$. However, how well the greedy algorithm works compared to `universal optimality' is still unclear for superconstant $k:=k(n)$. In this paper, we aim to give a new and fine-grained analysis of this problem in undirected unweighted graph setting. Specifically, we show some bounds on this problem including the following two (1) On the negative side, when $k<\frac{1}{3}n-O(1)$, the greedy algorithm is not `universally optimal'. (2) On the positive side, when $k>\frac{2}{3}n+O(1)$, the greedy algorithm is `universally optimal'. We also introduce an appropriate notion for `approximately universal optimality'. An algorithm is $(\alpha,\beta)$-universally optimal iff given any $n$-vertex input graph $G$, it can produce a $k$-spanner $H$ of $G$ with size $|H|\leq n+\alpha(|H^*|-n)+\beta$, where $H^*$ is the smallest $k$-spanner of $G$. We show the following positive bounds. (1) When $k>\frac{4}{7}n+O(1)$, the greedy algorithm is $(2,O(1))$-universally optimal. (2) When $k>\frac{12}{23}n+O(1)$, the greedy algorithm is $(18,O(1))$-universally optimal. (3) When $k>\frac{1}{2}n+O(1)$, the greedy algorithm is $(32,O(1))$-universally optimal. All our proofs are constructive building on new structural analysis on spanners. We give some ideas about how to break small cycles in a spanner to increase the girth. These ideas may help us to understand the relation between girth and spanners.