The city of Detroit and other private-public partnerships have attempted to catalyze the region's growth by facilitating the building and historical rehabilitation of residential high-rises in the downtown, creating a zone that offers many business tax incentives, creating recreational spaces such as the Detroit RiverWalk, Campus Martius Park, Dequindre Cut Greenway, and Green Alleys in Midtown. Which part of Detroit has gotten billions in investment around the turn of the millennium