J. Verdera, ON THE UNIFORM APPROXIMATION PROBLEM FOR THE SQUARE OF THE CAUCHY-RIEMANN OPERATOR, Pacific journal of mathematics, 159(2), 1993, pp. 379-396
Let X be a compact subset of the plane and f a continuous function on
X satisfying the equation partial derivative2BAR f = 0 in the interior
of X . It is unknown whether f can be uniformly approximated on X by
functions g satisfying the equation partial derivative2BAR g = 0 in so
me neighbourhood (depending on g) of X. We show that this is the case
under the additional assumption that f satisfies a Dini-type continuit
y condition.