The Integral Trees (Larry Niven): the habitable space is a torus. It's a torus of air in orbit. The trees look like integral signs because they align pointed t

The Integral Trees (Larry Niven): the habitable space is a torus. It's a torus of air in orbit. The trees look like integral signs because they align pointed toward the star, but have constant wind in opposite directions at either end, because the air there has different orbital speeds, being closer or further from the star.