arXiv subjects

Tomoya Mori

Publications and source records attributed to Tomoya Mori.

10 recordsLinked to original sources

arXiv2026

How an Economy Shrinks in Space: Concavity-on-Jobs and Upward Consolidation under Demographic Decline

When a country's population declines, the aggregate economy appears to contract on the intensive margin: industrial diversity intact, every industry a little smaller. At the regional level, contraction is uneven and takes the extensive form: entire industries disappear, one after another. The relevant unit is the city: industries are nested by size - the hierarchy property of industrial location - each viable only above a minimum population. Necessity industries' thresholds bunch at the low end, so a city's industry count - and its jobs - is sharply concave in size (concavity on jobs). A modest loss pushes a small city below many thresholds at once; a large core sheds a few specialized industries, one at a time. Lost industries consolidate upward to the next city large enough to host them; for the worker it means a step down to a lower-paid local job. To recover that income, workers move up to the apex - the only city hosting the full industry range. Studying Japan - two decades ahead of the OECD, Tokyo at its apex - with worker-level panel data on the young workers who carry the migration, a wage regression in real, housing-inclusive wages identifies a Tokyo-bound migration incentive that varies by origin, following concavity on jobs.

econ.GN↗

arXiv2026

Spatial scale of agglomeration and dispersion: Number, spacing, and the spatial extent of cities

How does transport cost affect the spatial organization of economic activities? This study develops a theoretical framework that distinguishes between two types of dispersion forces in spatial models: "local" dispersion forces acting within cities, and "global" dispersion forces acting across them. The distinction leads to a systematic classification of spatial models into a few fundamental types, each with distinct endogenous spatial patterns and comparative statics in response to changes in transport costs. The framework reconciles empirical findings and clarifies how transport-induced reorganization of economic activities can depend on the spatial scale of dominant dispersion forces.

econ.GN↗

arXiv2025

Sustainability of cities under declining population and decreasing distance frictions: The case of Japan

This study develops a statistical model that integrates economic agglomeration theory and power-law distributions of city sizes to project future population distribution on 1-km grid cells. We focus on Japan -- a country at the forefront of rapid population decline. Drawing on official population projections and empirical patterns from past urban evolution in response to the development of high-speed rail and highway networks, we examine how ongoing demographic contraction and expected reductions in distance frictions may reshape urban geography. Our analysis suggests that urban economies will consolidate around fewer and larger cities, each of which will experience a flattening of population density as the decentralization of urban populations accelerates, while rural areas are expected to experience further depopulation as a result of these spatial and economic shifts. By identifying sustainable urban cores capable of anchoring regional economies, our model provides a framework for policymakers to manage population decline while maintaining resilience through optimized infrastructure and resource allocation focused on these key urban centers.

econ.GN↗

arXiv2023

Origin of power laws and their spatial fractal structure for city-size distributions

City-size distributions follow an approximate power law in various countries despite high volatility in relative city sizes over time. Our empirical evidence for the United States and Japan indicates that the scaling law stems from a spatial fractal structure owing to the coordination of industrial locations. While the locations of individual industries change considerably over time, there is a persistent pattern in that more localized industries at a given time are found only in larger cities. The spatial organization of cities exhibits a hierarchical structure in which larger cities are spaced apart to serve as centers for surrounding smaller cities, generating a recursive pattern across different spatial scales. In our theoretical replication of the observed regularities, diversity in scale economy among industries induces diversity in their location pattern, which translates into diversity in city size via spatial coordination of industries and population. The city-size power law is a generic feature of Monte-Carlo samples of stationary states resulting from the spontaneous spatial fractal structure in the hypothetical economy. The identified regularities reveal constraints on feasible urban planning at each regional scale. The success or failure of place-based policies designed to take advantage of individual cities' characteristics should depend on their spatial relationships with other cities, subject to the nationwide spatial fractal structure.

econ.GN↗

arXiv2022

Collaborative knowledge exchange promotes innovation

Considering collaborative patent development, we provide micro-level evidence for innovation through exchanges of differentiated knowledge. Knowledge embodied in a patent is proxied by word pairs appearing in its abstract, while novelty is measured by the frequency with which these word pairs have appeared in past patents. Inventors are assumed to possess the knowledge associated with patents in which they have previously participated. We find that collaboration by inventors with more mutually differentiated knowledge sets is likely to result in patents with higher novelty.

econ.GN↗

arXiv2021

Tree Edit Distance with Variables. Measuring the Similarity between Mathematical Formulas

In this article, we propose tree edit distance with variables, which is an extension of the tree edit distance to handle trees with variables and has a potential application to measuring the similarity between mathematical formulas, especially, those appearing in mathematical models of biological systems. We analyze the computational complexities of several variants of this new model. In particular, we show that the problem is NP-complete for ordered trees. We also show for unordered trees that the problem of deciding whether or not the distance is 0 is graph isomorphism complete but can be solved in polynomial time if the maximum outdegree of input trees is bounded by a constant. This distance model is then extended for measuring the difference/similarity between two systems of differential equations, for which results of preliminary computational experiments using biological models are provided.

cs.DS↗

arXiv2021

Cities in a world of diminishing transport costs

Economic activities favor mutual geographical proximity and concentrate spatially to form cities. In a world of diminishing transport costs, however, the advantage of physical proximity is fading, and the role of cities in the economy may be declining. To provide insights into the long-run evolution of cities, we analyzed Japan's census data over the 1970--2015 period. We found that fewer and larger cities thrived at the national scale, suggesting an eventual mono-centric economy with a single megacity; simultaneously, each larger city flattened out at the local scale, suggesting an eventual extinction of cities. We interpret this multi-scale phenomenon as an instance of pattern formation by self-organization, which is widely studied in mathematics and biology. However, cities' dynamics are distinct from mathematical or biological mechanisms because they are governed by economic interactions mediated by transport costs between locations. Our results call for the synthesis of knowledge in mathematics, biology, and economics to open the door for a general pattern formation theory that is applicable to socioeconomic phenomena.

econ.GN↗

arXiv2020

Creation of knowledge through exchanges of knowledge: Evidence from Japanese patent data

This study shows evidence for collaborative knowledge creation among individual researchers through direct exchanges of their mutual differentiated knowledge. Using patent application data from Japan, the collaborative output is evaluated according to the quality and novelty of the developed patents, which are measured in terms of forward citations and the order of application within their primary technological category, respectively. Knowledge exchange is shown to raise collaborative productivity more through the extensive margin (i.e., the number of patents developed) in the quality dimension, whereas it does so more through the intensive margin in the novelty dimension (i.e., novelty of each patent).

econ.GN↗

arXiv2019

Spatial pattern and city size distribution

Many large cities are found at locations with certain first nature advantages. Yet, those exogenous locational features may not be the most potent forces governing the spatial pattern of cities. In particular, population size, spacing and industrial composition of cities exhibit simple, persistent and monotonic relationships. Theories of economic agglomeration suggest that this regularity is a consequence of interactions between endogenous agglomeration and dispersion forces. This paper reviews the extant formal models that explain the spatial pattern together with the size distribution of cities, and discusses the remaining research questions to be answered in this literature. To obtain results about explicit spatial patterns of cities, a model needs to depart from the most popular two-region and systems-of-cities frameworks in urban and regional economics in which there is no variation in interregional distance. This is one of the major reasons that only few formal models have been proposed in this literature. To draw implications as much as possible from the extant theories, this review involves extensive discussions on the behavior of the many-region extension of these models. The mechanisms that link the spatial pattern of cities and the diversity in city sizes are also discussed in detail.

econ.GN↗

arXiv2019

Cities and space: Common power laws and spatial fractal structures

City size distributions are known to be well approximated by power laws across a wide range of countries. But such distributions are also meaningful at other spatial scales, such as within certain regions of a country. Using data from China, France, Germany, India, Japan, and the US, we first document that large cities are significantly more spaced out than would be expected by chance alone. We next construct spatial hierarchies for countries by first partitioning geographic space using a given number of their largest cities as cell centers, and then continuing this partitioning procedure within each cell recursively. We find that city size distributions in different parts of these spatial hierarchies exhibit power laws that are again far more similar than would be expected by chance alone -- suggesting the existence of a spatial fractal structure.

econ.GN↗