English

Comparing the numbers of subforests and subgraph-degree-tuples

Combinatorics 2025-11-03 v1

Abstract

We enumerate the row-column-sums of all square tridiagonal (0,1)(0,1)-matrices and prove that their count coincides with OEIS A022026 - the number of acyclic subgraphs of the complete 2×n2\times n grid graph. We then extend this correspondence in two independent directions: 1. admitting larger sets of matrix entries, and 2. relaxing the tridiagonal support to broader prescribed sparsity patterns. The latter leads us to conjecture that, for any bipartite graph GG, the number of its acyclic subgraphs equals the number of degree sequences realized by subgraphs of GG. Moreover, for any non-bipartite graph, the former should be strictly smaller than the latter. We discuss several general approaches and prove these hypotheses for cactus graphs and generalized book graphs.

Keywords

Cite

@article{arxiv.2510.26936,
  title  = {Comparing the numbers of subforests and subgraph-degree-tuples},
  author = {Sergei Shteiner and Pavel Shteyner},
  journal= {arXiv preprint arXiv:2510.26936},
  year   = {2025}
}

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24 pages