  In this paper we prove existence and uniqueness of an entropy solution to the
A-obstacle problem, for L^1 data. We also extend the Lewy-Stampacchia
inequalities to the general framework of L^1 data, and show convergence and
stability results. We then prove that the free boundary has finite N-1
Hausdorff measure, which completes previous works on this subject by Caffarelli
for the Laplace operator and by Lee and Shahgholian for the p-Laplace operator
when p>2.
