  As further development of earlier works on the $(f,g)$-inversion, the present
paper is devoted to the $(f,g)$-difference operator and the representation
problem or an expansion formula of analytic functions.
  A recursive formula and the Leibniz formula for the $(f,g)$-difference
operator of the product of two functions are established.
  The resulting expansion formula not only unifies the $q$-analogue of the
Lagrange inversion formula of Gessel and Stanton (thus, a $q$-expansion formula
of Liu) for $q$-series but also systematizes the "Ismail's argument". In the
meantime, a rigorous analytic proof of the $(1-xy,x-y)$-expansion formula with
respect to geometric series, along with a proof of the previously unknown fact
that it is equivalent to a $q$-analogue of the Lagrange inversion formula due
to Gessel and Stanton, is presented. As applications, new proofs of several
well-known summation and transformation formulas are investigated.
