Recent work on “quiet planting” in combinatorial optimization aims to generate instances with a hidden solution that is hard to recover, typically by making the planted distribution statistically indistinguishable from uniform for specific algorithms, such as statistical queries. A prominent example is planted $k$-SAT, where $O(n^{k/2})$ clauses can be planted while maintaining indistinguishability from uniform instances, evidenced by prior hardness results which also align with findings in SAT refutation. Despite extensive research and practical use in benchmarking SAT solvers, the challenge of quietly planting multiple solutions while preserving hardness has remained an open problem. This work initiates the study of quiet planting with an arbitrary number of solutions, proposing the first method to construct quiet planting distributions for $k$-SAT formulas that accommodate more than one solution. We provide statistical query lower bounds for distinguishing these planted instances from uniform ones, and our method allows for planting solutions with arbitrary geometric relationships, including varying Hamming distances. A key innovation facilitating multiple solutions is the ability to incorporate arbitrary correlations between variable selection in clauses and their negation patterns, departing from prior approaches. We also investigate the worst-case complexity of SAT by showing the difficulty in distinguishing satisfiable instances with numerous solutions from unsatisfiable ones, addressing an open problem of Hsieh, Mohanty, and Xu (CCC’22). From a technical standpoint, we generalize the concept of $(r-1)$-wise uniformness in clause distributions, proving hardness holds if the marginal distribution over negation patterns is $(r-1)$-wise uniform, and reveal a connection to binary linear codes, demonstrating how a $[k, t, r]$ code can guide the planting of up to $2^t - 1$ solutions on $k$ variables with $(r-1)$-wise uniform negation distributions.

The Steiner Forest problem, also known as the Generalized Steiner Tree problem, is a fundamental optimization problem on edge-weighted graphs where, given a set of vertex pairs, the goal is to select a minimum-cost subgraph such that each pair is connected. This problem generalizes the Steiner Tree problem, first introduced in 1811, for which the best known approximation factor is 1.39 by [Byrka, Grandoni, Rothvoβ, and Sanità, 2010]. The celebrated work of [Agrawal, Klein, and Ravi, 1989], along with refinements by [Goemans and Williamson, 1992], established a 2-approximation for Steiner Forest over 35 years ago. Pioneering iterative rounding techniques by [Jain, 1998] later extended these results to higher connectivity settings. Despite the long-standing importance of this problem, breaking the approximation factor of 2 has remained a major challenge, raising suspicions that achieving a better factor might indeed be hard. In this paper, we break the approximation barrier of 2 by designing a novel deterministic algorithm that achieves a $\mathbf{2} \mathbf{- 1 0}^{\mathbf{- 1 1}}$ approximation for this fundamental problem. As a key component of our approach, we also introduce a novel dual-based local search algorithm for the Steiner Tree problem with an approximation guarantee of 1.943, which is of independent interest.

Approximation algorithms for the prize-collecting Steiner forest problem (PCSF) have been a subject of research for over three decades, starting with the seminal works of Agrawal, Klein, and Ravi [1, 2] and Goemans and Williamson [15, 16] on Steiner forest and prize-collecting problems. In this paper, we propose and analyze a natural deterministic algorithm for PCSF that achieves a 2-approximate solution in polynomial time. This represents a significant improvement compared to the previously best known algorithm with a 2.54-approximation factor developed by Hajiaghayi and Jain [20] in 2006. Furthermore, Könemann, Olver, Pashkovich, Ravi, Swamy, and Vygen [25] have established an integrality gap of at least 9/4 for the natural LP relaxation for PCSF. However, we surpass this gap through the utilization of an iterative algorithm and a novel analysis technique. Since 2 is the best known approximation guarantee for Steiner forest problem [2] (see also [16]), which is a special case of PCSF, our result matches this factor and closes the gap between the Steiner forest problem and its generalized version, PCSF.

Approximation algorithms for the prize-collecting Steiner forest problem (PCSF) have been a subject of research for over three decades, starting with the seminal works of Agrawal, Klein, and Ravi [1, 2] and Goemans and Williamson [14, 15] on Steiner forest and prize-collecting problems. In this paper, we propose and analyze a natural deterministic algorithm for PCSF that achieves a 2-approximate solution in polynomial time. This represents a significant improvement compared to the previously best known algorithm with a 2.54-approximation factor developed by Hajiaghayi and Jain [19] in 2006. Furthermore, Könemann, Olver, Pashkovich, Ravi, Swamy, and Vygen [24] have established an integrality gap of at least 9/4 for the natural LP relaxation for PCSF. However, we surpass this gap through the utilization of a combinatorial algorithm and a novel analysis technique. Since 2 is the best known approximation guarantee for Steiner forest problem [2] (see also [15]), which is a special case of PCSF, our result matches this factor and closes the gap between the Steiner forest problem and its generalized version, PCSF.

Prize-Collecting Steiner Tree (PCST) is a generalization of the Steiner Tree problem, a fundamental problem in computer science. In the classic Steiner Tree problem, we aim to connect a set of vertices known as terminals using the minimum-weight tree in a given weighted graph. In this generalized version, each vertex has a penalty, and there is flexibility to decide whether to connect each vertex or pay its associated penalty, making the problem more realistic and practical.