DOI: 10.68381/jca32064 ISSN: 0944-6532

On Diversities and Finite Dimensional Banach Spaces

Bernardo González Merino

A diversity

\delta δ
in
M M
is a function defined over every finite set of points of
M M
mapped onto
[0,\infty) [ 0 , ∞ )
, with the properties that
\delta(X)=0 δ ( X ) = 0
if and only if
|X|\leq 1 ∣ X ∣ ≤ 1
and
\delta(X\cup Y)\leq\delta(X\cup Z)+\delta(Z\cup Y) δ ( X ∪ Y ) ≤ δ ( X ∪ Z ) + δ ( Z ∪ Y )
, for every finite sets
X,Y,Z\subset M X , Y , Z ⊂ M
with
|Z|\geq 1 ∣ Z ∣ ≥ 1
. Its importance relies in the fact that, amongst others, they generalize the notion of metric distance. We characterize when a diversity
\delta δ
defined over
M M
,
|M|=3 ∣ M ∣ = 3
, is Banach-embeddable, i.e. when there exist points
p_i p i
,
i=1,2,3 i = 1 , 2 , 3
, and a symmetric, convex, and compact set
C C
such that
\delta(\{x_{i_1},\dots,x_{i_m}\})=R(\{p_{i_1},\dots,p_{i_m}\},C) δ ( { x i 1 , … , x i m } ) = R ( { p i 1 , … , p i m } , C )
, where
R(X,C) R ( X , C )
denotes the circumradius of
X X
with respect to
C C
. Moreover, we also characterize when a diversity
\delta δ
is a Banach diversity, i.e. when
\delta(X)=R(X,C) δ ( X ) = R ( X , C )
, for every finite set
X\subset\mathbb R^n X ⊂ R n
, where
C C
is an
n n
-dimensional, symmetric, convex, and compact set.