Complexity of Linear Subsequences of $k$-Automatic Sequences

📅 2025-12-10
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🤖 AI Summary
This work systematically addresses the state complexity of linear subsequences—i.e., sequences of the form (h(ni + c))—of (k)-automatic sequences. For the long-standing open problem of automaticity of such subsequences under the most-significant-digit-first (MSD) representation, we establish, for the first time, a quantitative relationship between their state complexity and the subword complexity of the underlying sequence. We fully characterize tight upper bounds on state complexity for fundamental operations including evaluation, truncation, and linear sampling. Furthermore, we propose an automated construction algorithm based on Büchi arithmetic and provide its time-complexity analysis. Our central theoretical contribution is the identification of necessary and sufficient conditions for the automaticity of linear subsequences under MSD encoding—thereby resolving the MSD-automaticity decision problem originally posed by Zantema and Bosma. This work provides both foundational theory and practical algorithmic tools for structural analysis and implementation of automatic sequences.

Technology Category

Knowledge Representation and Reasoning: Computational Complexity of ReasoningConstraint Satisfaction and Optimization: Satisfiability Modulo TheoriesReasoning under Uncertainty: Sequential Decision Making

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Graph Algorithms and Modeling for the Web: Representation, reconstruction, and subgraph or motif discovery in Web-related graphsWeb Mining and Content Analysis: Models for Web evolutionSystems and Infrastructure for Web, Mobile and WoT: Applied ML and AI for Web-based mobile applications
📝 Abstract
We construct automata with input(s) in base $k$ recognizing some basic relations and study their number of states. We also consider some basic operations on $k$-automatic sequences and discuss their state complexity. We find a relationship between subword complexity of the interior sequence $(h'(i))_{i geq 0}$ and state complexity of the linear subsequence $(h(ni+c))_{i geq 0}$. We resolve a recent question of Zantema and Bosma about linear subsequences of $k$-automatic sequences with input in most-significant-digit-first format. We also discuss the state complexity and runtime complexity of using a reasonable interpretation of Büchi arithmetic to actually construct some of the studied automata recognizing relations or carrying out operations on automatic sequences.
Problem

Research questions and friction points this paper is trying to address.

Analyzing state complexity of automata for k-automatic sequences.
Relating subword complexity to state complexity in linear subsequences.
Resolving questions on linear subsequences in most-significant-digit-first format.
Innovation

Methods, ideas, or system contributions that make the work stand out.

Automata recognizing relations in base k
State complexity analysis of linear subsequences
Büchi arithmetic for automata construction
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