Denoting by E⊆R2 the set of the pairs
(λ1(Ω),λ2(Ω)) for all the open sets
Ω⊆RN with unit measure, and by Θ⊆RN the union
of two disjoint balls of half measure, we give an elementary proof of the fact
that \partial\E has horizontal tangent at its lowest point
(λ1(Θ),λ2(Θ)).Comment: 7 pages, 3 figure