On Linear Transformations of Intersections


For any linear transformation and two convex closed sets, we provide necessary and sufficient conditions for the transformation of the intersection of the sets to coincide with the intersection of their images. We also identify conditions for non-convex closed sets, continuous transformations, and multiple sets. We demonstrate the usefulness of our results via an application to the economics literature of mechanism design.

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We are very grateful to Itai Ashlagi, Egon Balas, Heinz Bauschke, Jérôme Bolte, Boris Bukh, Keenan Crane, Patrick Combettes, Gerard Cornuejols, Federico Echenique, Alfred Galichon, Ben Golub, Sergiu Hart, Fatma Kılınç-Karzan, Michael McCoy, Javier Pena, Marek Pycia, R Ravi, Stephen Spear, Rakesh Vohra, Josephine Yu, Weijie Zhong, two anonymous referees, and seminar participants at Columbia University, Carnegie Mellon University, Higher School of Economics, New Economic School, and the University of Pittsburgh for discussions and useful suggestions. Shuo Liu acknowledges the financial support by the Forschungskredit of the University of Zurich, grant no. FK-17-018.

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Theorem 1

Consider two convex closed sets\(A, B\subset \mathbb {R}^{n}\). If for all linear transformations\(T:\mathbb {R}^{n}\rightarrow \mathbb {R}^{n-1}\)we have T(AB) = TATB, then AB is convex.


Take some aA and bB such that ab, and consider any linear transformation \(T: \mathbb {R}^{n}\rightarrow \mathbb {R}^{n-1}\) with \(\ker (T)=\{v\in \mathbb {R}^{n}: v=k\cdot (a-b), k\in \mathbb {R}\}\). By construction, the kernel of T amounts to the lines spanned by ab, and we have \(a-b\in \ker (T)\) and Ta = Tb = t for some \(t\in \mathbb {R}^{n-1}\). As tTATB = T(AB), there exists cAB such that Tc = t. As \(\dim (\ker (T))=1\), points a, b, c must lie on one straight line. As A and B are convex and cAB, we have [a, c] ⊂ A and [b, c] ⊂ B, which implies that [a, b] ⊂ AB. Hence, AB is convex. □

Proof of Theorem 8

We want to prove that if \(\forall d\in \ker (T)^{\perp }~E(A\cap B, d)\subseteq E(A, d)\cup E(B, d)\), then T(AB) = TATB. As T(AB) ⊂ TATB and both T(AB) and TATB are convex compact sets, it suffices to show that E(T(AB), d) ⊂ E(TATB, d) for all directions \(d\in \mathbb {R}^{m}\), that is, every support point of T(AB) is also a support point of TATB in the same direction. For this purpose, consider any direction \(d\in \mathbb {R}^{m}\). Let \(T^{*}: \mathbb {R}^{m}\rightarrow \mathbb {R}^{n}\) be the adjoint operator (or the transpose) of T. As \(\text {image}(T^{*})=\ker (T)^{\perp }\) (see p. 120 in [2]) we have \(T^{*}(d)\in \ker (T)^{\perp }\). The condition of the theorem then ensures that \(E(A\cap B, T^{*}(d))\subseteq E(A, T^{*}(d))\cup E(B, T^{*}(d))\). By the definition of adjoint operators, we further have \( E(T(A\cap B), d)\subseteq E(TA, d)\cup E(TB, d). \)

Now consider any \(t\in E(T(A\cap B), d)\subseteq E(TA, d)\cup E(TB, d)\). Without loss, we suppose that tE(TA, d). T(AB) ⊂ TATB implies that tTATB, and since TATB is a subset of TA, from the relation tE(TA, d) we can conclude that tE(TATB, d). □

Proof of Theorem 9

We want to prove that T(AB) = TATB if and only if \(\forall d\in \ker (T)^{\perp }\) and ∀uE(AB, d), \(\exists d^{\prime }, d^{\prime \prime }\in \ker (T)^{\perp }\) such that \(d^{\prime }+d^{\prime \prime }=d\) and \(u\in E(A, d^{\prime })\cap E(B, d^{\prime \prime })\).

(If statement) For arbitrary direction \(z\in \mathbb {R}^{m}\) the support function of the image of the intersection equals

$$ S^{T(A\cap B)}(z)=\sup_{t\in T(A\cap B)} t\cdot z=\sup_{x\in A\cap B} x\cdot T^{*}(z)=S^{A\cap B}(T^{*}(z)), $$

where T is the adjoint operator. As \(\text {image}(T^{*})=\ker (T)^{\perp }\) we have \(T^{*}(z)\in \ker (T)^{\perp }\). The support function for the intersection of convex sets having non-empty intersection of their relative interiors (riA ∩riB) can be conveniently characterized by (see Corollary 16.4.1 in [17])

$$ S^{A\cap B}(T^{*}(z))=\underset{d^{\prime},d^{\prime\prime} \in \mathbb{R}^{n} }{\underset{d^{\prime}+d^{\prime\prime}=T^{*}(z)}{\inf}} (S^{A}(d^{\prime})+S^{B}(d^{\prime\prime})). $$

Since set AB is compact, the set of support points E(AB, T(z)) is non-empty. The condition of the theorem then implies that for any uE(AB, T(z)), there exist \(d^{\prime }, d^{\prime \prime }\in \ker (T)^{\perp }\), such that \(d^{\prime }+d^{\prime \prime }=T^{*}(z)\) and \(u\in E(A, d^{\prime })\cap E(B, d^{\prime \prime })\). Therefore, we have

$$ S^{A\cap B}\left( T^{*}(z)\right)=u\cdot T^{*}(z)=u\cdot d^{\prime}+u\cdot d^{\prime\prime}=S^{A}(d^{\prime})+S^{B}(d^{\prime\prime}). $$

Equations (1) and (2) then imply that

$$ S^{A\cap B}(T^{*}(z))= \underset{d^{\prime},d^{\prime\prime} \in \ker(T)^{\perp}}{\underset{d^{\prime}+d^{\prime\prime}=T^{*}(z)}{\inf}} (S^{A}(d^{\prime})+S^{B}(d^{\prime\prime})). $$

As \(d^{\prime },d^{\prime \prime }\in \ker (T)^{\perp }=\text {image}(T^{*})\) there exist \(z^{\prime }, z^{\prime \prime }\in \mathbb {R}^{m}\) such that \(d^{\prime }=T^{*}(z^{\prime })\), \(d^{\prime \prime }=T^{*}(z^{\prime \prime })\), and \(z^{\prime }+z^{\prime \prime }=z\).


$$ \begin{array}{@{}rcl@{}} S^{A\cap B}(T^{*}(z))&=&\inf_{{z^{\prime}+z^{\prime\prime}=z} \atop {z^{\prime},z^{\prime\prime} \in\mathbb{R}^{m} }} (S^{A}(T^{*}(z^{\prime}))+S^{B}(T^{*}(z^{\prime\prime})))\\ &=& \underset{z^{\prime},z^{\prime\prime} \in \mathbb{R}^{m} }{\underset{z^{\prime}+z^{\prime\prime}=z}{\inf}} (S^{TA}(z^{\prime})+S^{TB}(z^{\prime\prime}))=S^{TA\cap TB}(z). \end{array} $$

Overall, ST(AB)(z) = STATB(z) for all directions \(z\in \mathbb {R}^{m}\). Hence, T(AB) = TATB.

(Only-if statement) To establish the necessity part, we assume that T(AB) = TATB and consider any direction \(d\in \ker (T)^{\perp }\) and support point uE(AB, d). As \(\ker (T)^{\perp }=\text {image}(T^{*})\) there must exist \(z\in \mathbb {R}^{m}\) such that d = T(z). For this direction, we have

$$ S^{TA\cap TB}(z)=\underset{z^{\prime},z^{\prime\prime} \in \mathbb{R}^{m}}{\underset{z^{\prime}+z^{\prime\prime}=z}{\inf}} \left( S^{TA}(z^{\prime})+S^{TB}(z^{\prime\prime})\right)=\underset{z^{\prime},z^{\prime\prime} \in \mathbb{R}^{m} }{\underset{z^{\prime}+z^{\prime\prime}=z}{\inf}} \left( S^{A}(T^{*}(z^{\prime}))+S^{B}(T^{*}(z^{\prime\prime}))\right). $$

As T(AB) = TATB there must exist \(z^{\prime }, z^{\prime \prime }\in \mathbb {R}^{m}\) such that \(z^{\prime }+z^{\prime \prime }=z\), and

$$ u\cdot d=S^{A\cap B}(T^{*}(z))=S^{T(A\cap B)}(z)=S^{TA\cap TB}(z)=S^{A}(d^{\prime})+S^{B}(d^{\prime\prime}), $$

where \(d^{\prime }=T^{*}(z^{\prime })\), \(d^{\prime \prime }=T^{*}(z^{\prime \prime })\), and the last equality follows from Corollary 16.4.1 in [17], which asserts that the infimum of (4) is achieved when the relative interiors of the two sets have a point in common. Also, as uAB, we have \(u\cdot d^{\prime }\leq S^{A}(d^{\prime })\) and \(u\cdot d^{\prime \prime }\leq S^{B}(d^{\prime \prime })\). As a result, we must have \( u\cdot d^{\prime }=S^{A}(d^{\prime }), ~\text {and}~u\cdot d^{\prime \prime }=S^{B}(d^{\prime \prime }). \) In other words, \(u\in E(A, d^{\prime })\cap E(B, d^{\prime \prime })\), where \(d^{\prime }, d^{\prime \prime }\in \ker (T)^{\perp }\) and \(d^{\prime }+d^{\prime \prime }=d=T^{*}(z)\). As the choice of z is arbitrary and \(\text {image}(T^{*})=\ker (T)^{\perp }\) the only-if statement follows. □

Proof of Theorem 10

We want to prove that under the assumptions of the theorem, \(\overline {T(A\cap B)}=\overline {TA\cap TB}\) if and only if for any dker(T),

$$ \underset{d^{\prime},d^{\prime\prime} \in \mathbb{R}^{n} }{\underset{d^{\prime}+d^{\prime\prime}=d}{\inf}} (S^{A}(d^{\prime})+S^{B}(d^{\prime\prime})) = \underset{d^{\prime},d^{\prime\prime} \in \ker(T)^{\perp}}{\underset{d^{\prime}+d^{\prime\prime}=d}{\inf}} (S^{A}(d^{\prime})+S^{B}(d^{\prime\prime})). $$

The argument is similar to the proof of Theorem 9. Let us first consider the sufficiency part. For arbitrary direction \(z\in \mathbb {R}^{m}\), we have

$$ S^{T(A\cap B)}(z)=\sup_{t\in T(A\cap B)} t\cdot z=\sup_{x\in A\cap B} x\cdot T^{*}(z)=S^{A\cap B}(T^{*}(z)), $$

Given that \(T^{*}(z)\in \ker (T)^{\perp }\), the condition of the theorem implies

$$ S^{A\cap B}(T^{*}(z))=\underset{d^{\prime},d^{\prime\prime} \in \mathbb{R}^{n}}{\underset{d^{\prime}+d^{\prime\prime}=T^{*}(z)}{\inf}} (S^{A}(d^{\prime})+S^{B}(d^{\prime\prime}))=\underset{d^{\prime},d^{\prime\prime} \in \ker(T)^{\perp} }{\underset{d^{\prime}+d^{\prime\prime}=T^{*}(z)}{\inf}} (S^{A}(d^{\prime})+S^{B}(d^{\prime\prime})). $$

Similar to the proof of Theorem 9, as \(d^{\prime },d^{\prime \prime }\in \ker (T)^{\perp }=\text {image}(T^{*})\) there exist \(z^{\prime }, z^{\prime \prime }\in \mathbb {R}^{m}\) such that \(d^{\prime }=T^{*}(z^{\prime })\), \(d^{\prime \prime }=T^{*}(z^{\prime \prime })\), and \(z^{\prime }+z^{\prime \prime }=z\). Hence,

$$ \begin{array}{@{}rcl@{}} S^{A\cap B}(T^{*}(z))&=&\underset{z^{\prime},z^{\prime\prime} \in \mathbb{R}^{m} }{\underset{z^{\prime}+z^{\prime\prime}=z}{\inf}} (S^{A}(T^{*}(z^{\prime}))+S^{B}(T^{*}(z^{\prime\prime})))\\ &=&\underset{z^{\prime},z^{\prime\prime} \in \mathbb{R}^{m} }{\underset{z^{\prime}+z^{\prime\prime}=z}{\inf}} (S^{TA}(z^{\prime})+S^{TB}(z^{\prime\prime}))=S^{TA\cap TB}(z). \end{array} $$

Overall, ST(AB)(z) = STATB(z) for all directions \(z\in \mathbb {R}^{m}\). In contrast to Theorem 9, since T(AB) and TATB are not necessary closed, this only implies that \(\overline {T(A\cap B)}=\overline {TA\cap TB}\).

Let us now establish the necessity part. Assume that \(\overline {T(A\cap B)}=\overline {TA \cap TB}\). This implies that for any \(z\in \mathbb {R}^{m}\) we have ST(AB)(z) = STATB(z). We know that

$$ \begin{array}{@{}rcl@{}} S^{TA\cap TB}(z)&=&\underset{z^{\prime},z^{\prime\prime} \in \mathbb{R}^{m} }{\underset{z^{\prime}+z^{\prime\prime}=z}{\inf}} \left( S^{TA}(z^{\prime})+S^{TB}(z^{\prime\prime})\right)\\ &=&\underset{z^{\prime},z^{\prime\prime} \in \mathbb{R}^{m} }{\underset{z^{\prime}+z^{\prime\prime}=z}{\inf}} \left( S^{A}(T^{*}(z^{\prime}))+S^{B}(T^{*}(z^{\prime\prime}))\right). \end{array} $$

As \(\text {image}(T^{*})=\ker (T)^{\perp }\), there must exist \(d^{\prime }, d^{\prime \prime }\in \ker (T)^{\perp }\) such that \(d^{\prime }=T^{*}(z^{\prime }), d^{\prime \prime }=T^{*}(z^{\prime \prime })\). Since T is linear and bijective we also have that sets \(\{d^{\prime },d^{\prime \prime }\in ker(T)^{\perp } | d^{\prime }+d^{\prime \prime }=T^{*}(z)\}\) and \(\{d^{\prime },d^{\prime \prime }\in ker(T)^{\perp } | d^{\prime }=T^{*}(z^{\prime }), d^{\prime \prime }=T^{*}(z^{\prime \prime }), z^{\prime }+z^{\prime \prime }=z\}\) coincide. Therefore,

$$ S^{TA\cap TB}(z)=\underset{z^{\prime},z^{\prime\prime} \in \ker(T)^{\perp} }{\underset{z^{\prime}+z^{\prime\prime}=T^{*}(z)}{\inf}}\left( S^{A}(z^{\prime})+S^{B}(z^{\prime\prime})\right). $$

At the same time, we have

$$ S^{T(A\cap B)}(z)=S^{A\cap B}(T^{*}(z))=\underset{z^{\prime},z^{\prime\prime} \in \mathbb{R}^{n} }{\underset{z^{\prime}+z^{\prime\prime}=T^{*}(z)}{\inf}} \left( S^{A}(z^{\prime})+S^{B}(z^{\prime\prime})\right). $$

Finally, as image(T) = ker(T), we can conclude that for any dker(T),

$$ \underset{z^{\prime},z^{\prime\prime} \in \mathbb{R}^{n} }{\underset{z^{\prime}+z^{\prime\prime}=z}{\inf}} (S^{A}(z^{\prime})+S^{B}(z^{\prime\prime})) = \underset{z^{\prime},z^{\prime\prime} \in \ker(T)^{\perp} }{\underset{z^{\prime}+z^{\prime\prime}=z}{\inf}} (S^{A}(z^{\prime})+S^{B}(z^{\prime\prime})). $$

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Kushnir, A., Liu, S. On Linear Transformations of Intersections. Set-Valued Var. Anal 28, 475–489 (2020). https://doi.org/10.1007/s11228-019-00525-0

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  • Linear transformation
  • Convex closed set
  • Intersection
  • Directional convexity
  • Mechanism design
  • Dominant-strategy implementation
  • Bayesian implementation

Mathematics Subject Classification (2010)

  • 52A20
  • 52A35
  • 91A99