Need only to find a function H of R^2 so that . One of the ways to define such a function is as follows:
Let n(t) be the unit normal vector field on h(t). For t in [0,1] and small values of s, a parametrization of the plane near Im(h) is given by r(t,s)=h(t)+s*n(t). Since Im(h) is compact, the parametrization is well defined on [0,1]*[-d,d], where d is a small positive value. Define H on R^2 as H(t,s)=s, then .
Then rotate the function H to define the function f on R^3 as .