g(x)=x1/2 is not defined for x<0
f(x)=x2 and g(x)=x1/2 are inverses for x in [0,∞).
Other common inverses: sin/arcin and ex/ln x.
Any function the graph of which is symmetric tom y=x will be its own inverse:
f(x)=(1-x2)1/2 for x in [0,1], f(x)=-x both are self inverses.