Let S(p),0<p<1,be the class of meromorphic univalent functions
f(z)in the unit disk Dwith a simple pole at p and normalization f(0)
=f'(0)-1=0.
In this paper,we first give a method to constract variation of f within S(p)according to the constraction of variations a function in S. Then we set up the Schiffer differential equation
(nf'(n)/f(n)²P(f(η))=q(η) (η∈D\{p})
for the extremal functions of the class S(p)and apply the equation to
discuss some properties of extremal functions.
