The function #y-=f(u)=1"/"(1+1"/"u)# can be rewritten as #f(u)=u"/"(u+1)#.
From the rewritten form, it's clear that #u!=-1# as this would cause the denominator to be equal to zero. From the original form, we can see that a horizontal asymptote will occur at #y=1#. This is because as #urarroo#, #yrarr1#.