Takahashi served the science team in an unusual capacity. His title of mathematician was hardly sufficient description of what he did; he seemed to move from interest to interest, working with one group on one day and another the next. He was more than a generalist—he was a generalist with a specific purpose, to oversee the mathematical and statistical rigor of the various groups within the science team. That explained how he had come to work with Rimskaya on preliminary corridor theory; they had discussed the topic while Takahashi double-checked Rimskaya's population studies.

Thistledown City was astonishing, newer than Alexandria by two centuries; it had been built after the Stone's launch, incorporating designs not thought of until the inhabitants had had long experience with their environment. Here the Stone architects had allowed themselves complete freedom. Treating the chamber as a giant valley, they had strung cables from cap to cap and hung buildings from them in graceful curves. Taking advantage of the upward slope of the floor, they had built arched structures fully ten kilometers long, bands of steel and processed Stone material interacting in patterns of silver and white, casting soft-edged shadows over the neighborhoods below. Some of the structures rose to the very limits of the chamber's atmosphere; these were actually thicker at the top than the bottom, like golf tees.

Even empty, Thistledown City seemed alive. It would take only the merest suggestion of people to come to life, Patricia thought; a few hundred citizens, moving from building to building, dressed in outrageous clothes—colorful, flowing garments suited to the curves and vaults and arches, bright colors to contrast with the muted creams, whites and metallics of the city.

The main library was practically hidden beneath a sprawling annex of one of the smaller golf-tee structures. Takahashi had said it was within easy walking distance, so they strolled across plazas, over pedestrian bridges, alongside service roads that at one time would have teemed with traffic—mostly computer-controlled and unoccupied vehicles. "All the vehicles are gone," Takahashi said. "We only know what they looked like through the records. They must have been put to use in the exodus."

She tried to imagine tens of millions of Stoners—such a population could easily have been accommodated by Thistledown City alone—tooling off down the corridor in their robot cars.

The library entrance was a solid sheet of a material resembling black marble. As they approached, an amplified voice asked them to halt for identification. They stood for a full two minutes before being cleared to enter.

A broad half-ellipse flowed aside in the black expanse. Beyond waited the ubiquitous security team in gray and black, passing them through after more ritual. The interior of the library was fully illuminated; no additional strip lighting was necessary. "No circuit breakers in Thistledown," Takahashi said. "We're not even sure how the power gets to the lights, much less where it comes from."