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α: calibrated so average coauthorship-adjusted count equals average raw count
The concentration of US imports is studied from transaction-level data. Concentration has fallen in the typical industry, while it is stable by industry-origin. Falling concentration is driven by the extensive margin: the number of exporting firms has grown, and the number of exported products has fallen relatively more for top firms. Instead, average revenue per product of top firms has increased. Top firms are converging at the industry level, but diverging within country. Finally, rising concentration from an origin is associated with falling prices, foreign entry and industry growth. These facts suggest that international competition coexists with national concentration.