DOI: 10.68381/jca16024 ISSN: 0944-6532

On the Lower Semicontinuous Quasiconvex Envelope for Unbounded Integrands (II): Representation by Generalized Controls

Marcus Wagner

[For the first part of this paper see ESAIM, Control, Optimisation and Calculus of Variations.] Motivated by the study of multidimensional control problems of Dieudonné-Rashevsky type, e.g. nonconvex correspondence problems from image processing, we raise the question how to understand to notion of quasiconvexity for a continuous function

f f
with a convex body
\mathrm{K} \subset \mathbb{R}^{nm} K ⊂ R n m
instead of the whole space
\mathbb{R}^{nm} R n m
as the range of definition. Extending
f f
by
(+\infty) ( + ∞ )
to the complement
\mathbb{R}^{nm}\setminus\,\mathrm{K} R n m ∖   K
, the appropriate quasiconvex envelope turns out to be
f^{(qc)}(w) = \sup\, \bigl\{g(w)\, \big\vert\, g \colon \mathbb{R}^{nm} \to \mathbb{R} \cup \{(+\infty)\} f ( q c ) ( w ) = sup ⁡   { g ( w )   ∣   g  ⁣ : R n m → R ∪ { ( + ∞ ) }
quasiconvex and lower semicontinuous,
g(v) \le f(v)\ \forall v \in \mathbb{R}^{nm}\bigr\}. g ( v ) ≤ f ( v )   ∀ v ∈ R n m } .
In the present paper, we prove that
f^{(qc)} f ( q c )
admits a representation as
f^{(qc)}(w) = f ( q c ) ( w ) =
Min
\bigl\{\int_{\mathrm{K}} f(v)\,d\nu(v)\, \big\vert\, \nu \in \mathrm{S}^{(qc)}(w)\bigr\} \quad \forall w \in \mathrm{K} { ∫ K f ( v )   d ν ( v )   ∣   ν ∈ S ( q c ) ( w ) } ∀ w ∈ K
where the sets
\mathrm{S}^{(qc)} (w) S ( q c ) ( w )
are nonempty, convex, weak
^* ∗
-sequentially compact subsets of probability measures. This theorem, forming a natural counterpart to the author's previous results about the representation of
f^{(qc)} f ( q c )
in terms of Jacobi matrices, has been proven indispensable for the derivation of Jensens' integral inequality as well as of differentiability theorems for the envelope
f^{(qc)} f ( q c )
. The paper is mainly concerned with a detailed analysis of the set-valued map
\mathrm{S}^{(qc)} S ( q c )
, which will be explicitely described in terms of averages of generalized controls.