This paper deals with a constrained minimization problemI t= inf Q(u+tψ)=1,u∈H 1 0(Ω,R 3) ∫ Ω|u| 2 d x d y,where Ω is a bounded domain in R 2, and Q(v) is defined asQ(v)=∫ Ωv·(v x∧v y) d x d y.The following conclusions are proved: 1) for ψ∈H 1 0(Ω,R 3), if ψ x∧ψ y0, and ψ∈C 1,1 0(Ω,R 3), then I t→S( as t→0); 2) if u t achieves I t, then there is some x 0∈Ω such that |u t| 2t→0Sδ x 0 in the sense of measure, where S= inf ∫ R 2 |u| 2 d x d yu∈H 1 ...