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arXiv 2511.03388cs.CC
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单调有界深度公式复杂度的图同态多项式

Monotone Bounded Depth Formula Complexity of Graph Homomorphism Polynomials

Indian Institute of Technology Gandhinagar(印度理工学院甘地纳加尔分校)

Balagopal Komarath, Rohit Narayanan

AI总结:

研究通过袋状消除树定义了图同态和着色同构多项式的单调有界深度公式复杂度,证明其复杂度为Θ(n^{λ_Δ(H)}),并建立了单调电路与公式间的分离及深度层次。

摘要:

我们引入了袋状消除树,这是一种扩展经典消除树的图分解方法,用于完全刻画图同态和着色同构多项式的单调有界深度公式复杂度。具体地,我们证明这些多项式的Δ-乘积深度单调公式复杂度为Θ(n^{λ_Δ(H)}),其中λ_Δ(H)是H在BET深度Δ下的最小袋状消除树成本。此结果闭合了Komarath、Pandey和Rahul等人发起的计划中最后一个开放案例:已知单调电路复杂度(通过树宽/有界深度树宽)、单调ABP复杂度(通过路径宽/有界深度路径宽)和单调公式复杂度(通过树深度)的紧致大小刻画;本定理通过新的有界深度袋状消除树成本λ_Δ概念提供了缺失的有界深度公式刻画,完成所有三种模型在代数复杂性及其固定深度变体中的完整图景。作为应用,对于常数度多项式族,我们得出在每个固定乘积深度下单调电路与单调公式之间的几乎最优分离:存在一个可由O(N)大小单调电路(乘积深度Δ)计算的多项式族,需要Ω(N^{Δ/2})大小的单调公式(相同深度)(该指数在常数因子范围内最优)。我们还证明了严格的深度层次:对于每个Δ≥1和每个常数k≥2,存在一个常数度族,其可由O(s(N))大小的单调公式(乘积深度Δ)计算,但需要Ω(s(N)^k)大小的单调公式(乘积深度Δ-1)。

英文摘要:

We introduce baggy elimination trees, a novel graph decomposition that generalises the classical elimination trees underlying treedepth, and use them to give a complete characterisation of the monotone bounded-depth formula complexity of graph homomorphism and coloured isomorphism polynomials. Specifically, we prove that the $Δ$-product depth monotone formula complexity of these polynomials is $Θ\!\left(n^{λ_Δ(H)}\right)$, where $λ_Δ(H)$ is the minimum cost of a baggy elimination tree for $H$ at BET-depth~$Δ$. This result closes the last open case in the programme initiated by Komarath, Pandey and Rahul and continued by Bhargav, Chen, Curticapean and Dwivedi: tight size characterisations of monotone circuit complexity (via treewidth / bounded-depth treewidth), monotone ABP complexity (via pathwidth / bounded-depth pathwidth), and monotone formula complexity (via treedepth) were already known; our theorem supplies the missing bounded-depth formula characterisation via the new notion of bounded-depth baggy-elimination-tree cost $λ_Δ$, completing the picture for all three models in algebraic complexity and their fixed depth variants. As applications, for constant-degree polynomial families we derive an almost-optimal separation between monotone circuits and monotone formulas at every fixed product depth: there exists a family computable by $O(N)$-size monotone circuits of product depth $Δ$ that requires $Ω(N^{Δ/2})$-size monotone formulas of the same depth (and this exponent is optimal up to a constant factor). We also prove a strict depth hierarchy: for every $Δ\geq 1$ and every constant $k \geq 2$, there is a constant-degree family with $O(s(N))$-size monotone formulas of product depth $Δ$ that requires $Ω(s(N)^k)$-size monotone formulas of product depth $Δ- 1$.

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