Torricellian points in normed linear spaces
© Dragomir et al.; licensee Springer. 2013
Received: 25 February 2013
Accepted: 6 May 2013
Published: 22 May 2013
Given a set of n (distinct) points in a normed space, we consider the set of Torricellian points, that is, the set of points which minimises the sum of distances to the points in . We introduce the Torricellian functional associated to a set of distinct points , which calculates the sum of distances of a point x to the points in . The Torricellian point is defined as the infimum (over all vectors) of this functional. We discuss the existence of Torricellian points in reflexive normed spaces, non-expansive subspaces and evidently, inner product spaces. A case for collinear points is given and is utilised to characterise strict convexity. For a non-collinear case, it is shown that the set of Torricellian points contains a unique point when the space is strictly convex. However, we show that the uniqueness of Torricellian point of a non-collinear set does not characterise strict convexity. We consider a particular example of the Torricellian problem in a space endowed with the Taxicab geometry.
KeywordsFermat point Torricellian point characterisation of strictly convex spaces Taxicab geometry
In 1643, Fermat raised the following problem : This problem was first solved by Torricelli, whose result was published by his pupil Viviani in 1659. This (unique) point is often referred to as the Fermat point, Fermat-Torricellian point, or Torricellian point. In this text, we refer to it as the Torricellian point. The solution is as follows: if all angles of the triangle are less than , then the Torricellian point is the interior point from which each side subtends an angle of . If one of the angles is greater than , then the Torricellian point lies at the obtuse angled vertex.
‘Given three distinct points in the plane, find the (unique) point having the minimal sum of distances to these three points.’
A straightforward generalisation of this problem is as follows: given n points (), find which minimises the sum of distances to all the n points. It is a well-known result that when (case of collinear points), the Torricellian point coincides with the centre point of the set when n is odd, i.e. , where ; and is any point lying between and , where when n is even (cf. Dalla  and Simons ). Given the large number of literature, we refer the reader to the survey paper by Kupitz and Martini  and Section II.8 of the book by Boltyanski et al.  for further reading on this topic.
Torricelli’s problem can be defined in any metric spaces, but if we wanted to have nice properties in characterising Torricellian points, we would have to restrict ourselves to the case of normed linear spaces. The set of Torricellian points can be utilised to characterise strictly convex spaces and inner product spaces. In Martini et al. , a generalisation in Minkowski spaces is considered. It is shown that the Torricellian point is unique if and only if the Minkowski space is strictly convex. Dragomir et al. in  considered and solved Torricelli’s problem in a real inner product space of dimension greater than 1. A generalisation to n arbitrary points in a real inner product space is considered in Dragomir and Comǎnescu .
The aim of this paper is to consider the generalisation of the Torricellian points in the settings of normed spaces (cf. Section 2). We introduce the Torricellian functional associated to a set of distinct points , which calculates the sum of distances of a point x to the points in . The Torricellian point is defined as the infimum (over all vectors) of this functional. We discuss the existence of the Torricellian points in Section 3. We recall some known results from the papers by Veselý , in which the author discussed the Torricellian set in reflexive normed spaces; and also Papini and Puerto  for some results in Banach spaces. We also consider the existence of Torricellian point for a set of distinct points which spans a non-expansive subspace; and as a corollary, we consider the existence in inner product spaces which recaptures the results of Dragomir and Comǎnescu . In Section 4, we consider the case of collinear points in normed spaces; and in particular, strictly convex spaces and inner product spaces. In Section 5 we show that for the case of non-collinear points, the Torricellian point is unique in strictly convex spaces and inner product spaces. However, the uniqueness of Torricellian points does not characterise strict convexity. Finally, in Section 6, we consider an example of the Torricellian problem in spaces endowed with the Taxicab geometry.
1 Definitions and notation
This section serves as a reference point for definitions and notation that are used in the paper.
When , we say that T is Gâteaux differentiable and denote the derivative as VT.
are called the inferior and superior semi-inner products.
for all ;
for all and ;
if , ;
for all ;
the space is smooth, i.e. the norm is Gâteaux differentiable on iff for all ; or iff is linear in the first variable;
where or .
2 Generalisation of Torricellian points
will be called the Torricellian functional associated with . The main properties of this mapping can be summarised in the following proposition. We refer to Dragomir and Comǎnescu [, Proposition 1] for the proof.
T is nonlinear on X;
T is continuous on X in the norm topology;
T is nonnegative and ;
T is convex on X.
Concerning the Gâteaux derivative of T, we have the following results.
for all .
and this completes the proof. □
for all .
Remark 2.4 Corollary 2.3 also holds for any inner product space as it is a smooth space. This recaptures the results in Dragomir and Comǎnescu [, Proposition 3].
Concerning the strict convexity property of T, we have the following proposition. We refer to [, Proposition 2] for the proof.
Proposition 2.5 Let be a strictly convex normed linear space. If with is a set of non-collinear points in X, then T is strictly convex on X.
Now, we formally define the Torricellian points for a given set of points.
The set of all Torricellian points of will be denoted by .
We describe the main properties of for any in the next proposition.
Proposition 2.7 For any , we have that is a convex, closed and bounded subset of the normed linear space X.
for all , which shows that , i.e. the set is convex.
Now, let and put . Then which is a closed set as the mapping is continuous in the norm topology of X and is closed in ℝ.
and thus proves the boundedness of the set . □
We introduce the following definition.
Definition 2.8 The Torricellian point is called segmentally inferior relating to the set if there exist , , such that .
which completes the proof. □
Corollary 2.10 The points with cannot be segmentally inferior to the set .
which contradicts (4) and thus proves the corollary. □
Definition 2.11 Let X be a normed linear space and the subsets of distinct points and . The subsets and ℬ are isometrically equivalent if there exists a distance preserving a bijective function which satisfies .
Without loss of generality, we suppose that for all . With regards to this definition, we have the following propositions. We omit the proofs.
for are equivalent.
The proof follows by observing that the map , is isometric in X for all .
3 The existence of Torricellian points
We start with the following known result (cf. Papini and Puerto [, Proposition 2.3]).
Proposition 3.1 If X is a dual space, in particular, if X is reflexive, then for all a set of distinct points in X, the Torricellian set is nonempty in X.
Remark 3.2 Veselý  proved that for any non-reflexive Banach space, there is an equivalent norm such that for some sets of three points the Torricellian point does not exist.
Furthermore, Papini and Puerto [, Theorem 2.1] proved the following result.
Proposition 3.3 If , then for every set , the Torricellian set is nonempty in X.
In what follows, we present a result for the existence for normed linear spaces which are not necessarily reflexive. We start with the following theorem.
P is the identity of Y, i.e. for all ;
for all ,
for all . By definition, if , then for all . Now, let . Then and thus . On the other hand, which gives us for all , i.e. ; this completes the proof. □
The following corollary contains a sufficient condition for the existence of the Torricellian points.
Corollary 3.5 Let be a normed linear space and let be a system of distinct points in X. If the subspace spanned by the set is non-expansive in X, then is nonempty.
Proof Since is a finite dimensional space, it implies that is reflexive. Thus, by Proposition 3.1, ; and from Theorem 3.4 it follows that , which proves the corollary. □
The following theorem contains an example of non-expansive linear subspaces in inner product spaces.
Theorem 3.6 Let be an inner product space. If G is a Čebyševian linear subspace in X, i.e. every element has a unique best approximant in G, then G is non-expansive in X.
and thus we obtain the desired inequality , and the theorem is proved. □
The above theorem gives us the following result of existence for the Torricellian points (cf. Dragomir and Comǎnescu ).
Proposition 3.7 Let be an inner product space. Then, for all , a set of distinct points in X, is nonempty.
which proves the statement. □
4 The case of collinear points
In this section, we consider the Torricellian points for a set of collinear points in a normed space.
Without loss of generality, we assume that . The following proposition holds.
Proposition 4.2 Let be a real normed space and let be be collinear points. Then .
Consequently, for all , which shows that is the unique Torricellian point associated with . □
The following proposition holds.
Proposition 4.3 Let be a normed linear space and let be a set of 2k collinear distinct points in X. Then the interval is a subset of .
which shows that . □
is strictly convex.
- (ii)For every and for every , 2k collinearly distinct point in X, we have
For every distinct points and in X, we have .
Proof ‘(i) ⟹ (ii)’.
where (see Proposition 4.2), which contradicts the fact that x minimises the Torricellian map T. Hence , and for some nonnegative θ. It follows that .
‘(ii) ⟹ (iii)’. The proof of this implication is trivial, so we omit the details.
where . Let us observe that by the inequality (6) we deduce , which contradicts the hypothesis , hence is strictly convex. □
Corollary 4.5 Let be an inner product space and let be 2k collinearly distinct points in X. Then .
5 The uniqueness of Torricellian points for non-collinear sets
In this section, we recall and present results concerning uniqueness of the set of Torricellian points.
Proposition 5.1 Let be a normed linear space and let () be a set of non-collinear points in X. If the space is strictly convex, then contains at most one element.
which produces a contradiction and the theorem is thus proved. □
We remark that Proposition 5.1 is also proved by Papini and Puerto  in their Remark 3.1.
The following corollary recaptures the result in Dragomir and Comănescu [, Theorem 1].
Corollary 5.2 Let be an inner product space and let () be a set of non-collinear points in X. Then contains a unique point.
Proof The existence follows by Theorem 3.7, while the uniqueness follows by Theorem 5.1, taking into account that every inner product space is a strictly convex space. □
Martini et al.  showed that the uniqueness of the Torricellian point characterises strict convexity in Minkowski spaces. In the next proposition, we show that there exists a non-collinear set, for which the Torricellian point is unique, but the normed space is not strictly convex, thus showing that the uniqueness property does not characterise strict convexity. We note that the definition of collinear in  is metric dependent. Thus, Proposition 5.1 only generalises their result when the metric agrees with our definition of collinearity (cf. Definition 4.1).
Proposition 5.3 Let be a normed space which is not strictly convex. There exists a set of non-collinear distinct points in X such that has at most one element.
Proof Since X is not strictly convex, by the proof of Theorem 4.4 (‘(iii) ⇒ (i)’), there exist , distinct points such that . Let . We observe that , which implies that , , are non-collinear distinct points. We have . If , then and and consequently, . Hence, . □
Suppose we define the property to be the following: For all the sets of distinct points, the set has at most one element. We then ask the following question: If the property holds true in a normed space X, can we affirm that X is strictly convex? The answer is no, and is verified by Corollary 6.4 of Section 6. Furthermore, we ask the question: If we generalise the property to , where , for what values of n can we affirm that X is strictly convex? In the next example, we show that when holds true in a normed space X and n is an odd number, X is not necessarily strictly convex.
Example 5.4 If we consider the space (which is not strictly convex), then for all sets (where n is an odd number) of distinct points, has at most one element (cf. Section 6).
It remains as an open problem at this point for the case of , where n is an even number.
6 The Torricellian points in the Taxicab geometry
where is a permutation which satisfies the property : . We note that the set does not depend on the permutation σ with the property .
6.1 The case
where in which the points are not necessary distinct. Using similar arguments to those in Proposition 4.2, Proposition 4.3 and Theorem 4.4, we obtain the following result.
We have a unique Torricellian point if and only if has a unique point;
If n is odd, then the set of Torricellian points has a unique element.
6.2 The general case
By Theorem 6.1 we have the following.
If n is odd, then has a unique point;
If n is even, then has a unique point if and only if all the sets have a unique element.
We present two sets of Torricellian points in the space : the first has one element and the second has an infinite number of elements.
We consider the Torricelli problem in the settings of normed spaces. We recall some known results such as the existence of Torricellian point(s) in reflexive spaces. Our result shows that when the set spans a non-expansive linear subspace of a normed space, the Torricellian point(s) exists. This implies that the Torricellian point of a set always exists in inner product spaces.
Furthermore, we show that in any normed space, if all points in the set lie in a single straight line (collinear), then the Torricellian set is the singleton . When all the points in the set are collinear, the Torricellian set contains the segment . This result can be used to characterise strictly convex spaces. When the Torricellian set of the collinear set is equal to the segment , then the space must be a strictly convex space and vice versa. This implies that when the space is an inner product space, the Torricellian set of the collinear set is the segment .
We discuss the uniqueness of the Torricellian points for non-collinear sets. We show that the Torricellian point is unique when the space is strictly convex. However, we show the uniqueness does not characterise strict convexity. Lastly, we consider some examples of the Torricellian points in a normed space endowed with the Taxicab geometry.
Dan Comǎnescu has been supported by the grant of the Romanian National Authority for Scientific Research, CNCS UEFISCDI, project number PN-II- RU-TE-2011-3-0006.
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