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Conformally Invariant Variational Problems. - SAM

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If A is almost everywhere invertible, and if it has the bound<br />

‖A‖ L∞ (D 2 ) +‖A −1 ‖ L∞ (D 2 ) < +∞ ,<br />

(VIII.12)<br />

then u is a solution of the Schrödinger system (VIII.6) if and<br />

only if it satisfies the conservation law<br />

div(A∇u−B∇ ⊥ u) = 0 .<br />

(VIII.13)<br />

If (VIII.13) holds, then u ∈ W 1,p<br />

loc (D2 ,R m ) for any 1 ≤ p <<br />

+∞, and therefore u is Hölder continuous in the interior of D 2 ,<br />

C 0,α<br />

loc (D2 ) for any α < +∞ .<br />

✷<br />

We note that the conservation law (VIII.13), when it exists,<br />

generalizes the conservation laws previously encountered in the<br />

study of problems with symmetry, namely:<br />

1) In the case of the constant mean curvature equation, the<br />

conservation law (VII.1) is (VIII.13) with the choice<br />

A ij = δ ij ,<br />

and<br />

⎛<br />

B = ⎜<br />

⎝<br />

0 −Hu 3 Hu 2<br />

Hu 3 0 −Hu 1<br />

⎞<br />

⎟<br />

⎠<br />

−Hu 2 Hu 1 0<br />

2) In the case of S n -valued harmonic maps, the conservation<br />

law (VII.25) is (VIII.13) for<br />

A ij = δ ij ,<br />

and B = (B i j ) with ∇ ⊥ B i j = u i ∇u j −u j ∇u i .<br />

Theultimatepartofthissectionwillbedevotedtoconstructing<br />

A and B, for any given antisymmetric Ω, with sufficiently<br />

81

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