Perhaps you should try some spelling variations:
population center » population census, population censuses, population change
de communication » mobile communication, online communication
technology 2.2 » technology _, technology 1, technology 3
2.2 population » 2.1 population
4 developing » 3 developing
center case » centers case, century case, center based
case de » case des, case d, case d1
costs 4 » costs _, costs a, costs 4.2

You may be able to get more results by adjusting your search query.

• Adding a wildcard symbol may retrieve word variants: technology 2.2 population center case de communication costs 4 developing _ african.*.