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This two-minute animation shows changes in the last 500 years of historical rankings for the world's 10 most populous cities.

You could argue that the world’s biggest city has hit a sweet spot: a flatlining population, pervasive transit and little gentrification. But is ‘peak city’ even possible – and where does Tokyo go from here?

The city will remain the centerpiece of economic growth in the 21st century. Here's how we're using smart city technology to optimize them even further.