It is said, that the harmful alleles get eliminated from population over a period of time, yet sickel cell anaemia is persisting in human population. Why?

"The population of a metro city experiences fluctuations in its population desity over a period of time." If 'N' is the population density at the time 't' , write the population density at the time 't+1'.

"The population of a metro city experiences fluctuations in its population desity over a period of time." When does the population in metro city tend to decline?

"The population of a metro city experiences fluctuations in its population desity over a period of time." When does the population in a metro city tend to increase?