标题： 美国城市的相干性 显示英文标题

标题： The Coherence of US cities

摘要： 多样化经济对城市维持其增长和发展至关重要，但它们也成本高昂，因为多样化通常需要扩大城市的能力建设。 我们通过衡量城市所支持的经济活动的一致性来分析城市如何管理这一权衡，这里的一致性定义为随机抽样的城市中有生产性的单位之间的技术距离。 我们使用这个框架来研究美国城市系统在过去近两个世纪的发展，从1850年到现在。 为此，我们依赖于历史人口普查数据，涵盖了超过6亿条个人记录，以描述1850年至1940年间城市经济活动的情况，以及近几十年来的800万项专利记录，以及城市详细的职业和产业概况。 尽管在这170年期间美国经济地理发生了巨大变化，其城市系统的平均一致性保持不变。 此外，在不同的时间段、数据集和相关性测量中，一致性随着城市规模以完全相同的速度下降，这表明多样化受到城市规模以普遍方式控制的限制。 显示英文摘要

摘要： Diversified economies are critical for cities to sustain their growth and development, but they are also costly because diversification often requires expanding a city's capability base. We analyze how cities manage this trade-off by measuring the coherence of the economic activities they support, defined as the technological distance between randomly sampled productive units in a city. We use this framework to study how the US urban system developed over almost two centuries, from 1850 to today. To do so, we rely on historical census data, covering over 600M individual records to describe the economic activities of cities between 1850 and 1940, and 8 million patent records as well as detailed occupational and industrial profiles of cities for more recent decades. Despite massive shifts in the economic geography of the U.S. over this 170-year period, average coherence in its urban system remains unchanged. Moreover, across different time periods, datasets and relatedness measures, coherence falls with city size at the exact same rate, pointing to constraints to diversification that are governed by a city's size in universal ways.