LetΩbe a bounded domain ofRnand letDbe aclosedsubset ofΩ.ForΨ∈W1,2(Ω)suchthatΨ0onΩ[1-2],we setKΨ={w∈W01,2(Ω)|wΨonDinthesense ofW1,2(Ω)}.Ifaij(x)=aji(x)(i,j=1,2,…,n)∈L∞(Ω)andλ|ξ|2 aij(x)ξiξj,x∈Ω,ξ∈Rn,(1.1)The obstacle problems are to find a functionu∈KΨwhich mini mizesj(v)=∫ΩaijivjvdxoverKΨ.Main resultsFor the obstacle problems,the following results arewell known,whose proof can be foundin[3]and[4].Lemma 1.1(i)There exists a uniqueΓΨ∈KΨsuchthat∫ΩaijiΓΨjΓΨdx=wi∈nfK∫ΨΩaijiwjwdx.(ii)There exists a unique...