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In mathematics, the modulus of convexity and the characteristic of convexity are measures of "how convex" the unit ball in a Banach space is. In some sense, the modulus of convexity has the same relationship to the ε-δ definition of uniform convexity as the modulus of continuity does to the ε-δ definition of continuity.

Definitions

The modulus of convexity of a Banach space (X, ||·||) is the function δ : [0, 2] → [0, 1] defined by

\( {\displaystyle \delta (\varepsilon )=\inf \left\{1-\left\|{\frac {x+y}{2}}\right\|\,:\,x,y\in S,\|x-y\|\geq \varepsilon \right\},} \)

where S denotes the unit sphere of (X, || ||). In the definition of δ(ε), one can as well take the infimum over all vectors x, y in X such that ǁxǁ, ǁyǁ ≤ 1 and ǁx − yǁ ≥ ε.[1]

The characteristic of convexity of the space (X, || ||) is the number ε0 defined by

\( {\displaystyle \varepsilon _{0}=\sup\{\varepsilon \,:\,\delta (\varepsilon )=0\}.} \)

These notions are implicit in the general study of uniform convexity by J. A. Clarkson (Clarkson (1936); this is the same paper containing the statements of Clarkson's inequalities). The term "modulus of convexity" appears to be due to M. M. Day.[2]
Properties

The modulus of convexity, δ(ε), is a non-decreasing function of ε, and the quotient δ(ε) / ε is also non-decreasing on (0, 2].[3] The modulus of convexity need not itself be a convex function of ε.[4] However, the modulus of convexity is equivalent to a convex function in the following sense:[5] there exists a convex function δ1(ε) such that

\( {\displaystyle \delta (\varepsilon /2)\leq \delta _{1}(\varepsilon )\leq \delta (\varepsilon ),\quad \varepsilon \in [0,2].} \)

The normed space (X, ǁ ⋅ ǁ) is uniformly convex if and only if its characteristic of convexity ε0 is equal to 0, i.e., if and only if δ(ε) > 0 for every ε > 0.
The Banach space (X, ǁ ⋅ ǁ) is a strictly convex space (i.e., the boundary of the unit ball B contains no line segments) if and only if δ(2) = 1, i.e., if only antipodal points (of the form x and y = −x) of the unit sphere can have distance equal to 2.
When X is uniformly convex, it admits an equivalent norm with power type modulus of convexity.[6] Namely, there exists q ≥ 2 and a constant c > 0 such that

\( {\displaystyle \delta (\varepsilon )\geq c\,\varepsilon ^{q},\quad \varepsilon \in [0,2].} \)

Modulus of convexity of the L p {\displaystyle L^{p}} L^{p} spaces

The modulus of convexity is known for the L^p spaces.[7] If \( {\displaystyle 1<p\leq 2} \), then it satisfies the following implicit equation:

\( {\displaystyle \left(1-\delta _{p}(\varepsilon )+{\frac {\varepsilon }{2}}\right)^{p}+\left(1-\delta _{p}(\varepsilon )-{\frac {\varepsilon }{2}}\right)^{p}=2.} \)

Knowing that \( {\displaystyle \delta _{p}(\varepsilon +)=0,} \) one can suppose that \( {\displaystyle \delta _{p}(\varepsilon )=a_{0}\varepsilon +a_{1}\varepsilon ^{2}+\cdots }. \) Substituting this into the above, and expanding the left-hand-side as a Taylor series around \( \varepsilon=0, \) one can calculate the \( a_{i} \) coefficients:

\( {\displaystyle \delta _{p}(\varepsilon )={\frac {p-1}{8}}\varepsilon ^{2}+{\frac {1}{384}}(3-10p+9p^{2}-2p^{3})\varepsilon ^{4}+\cdots .} \)

For \( {\displaystyle 2<p<\infty }, one has the explicit expression

\( {\displaystyle \delta _{p}(\varepsilon )=1-\left(1-\left({\frac {\varepsilon }{2}}\right)^{p}\right)^{\frac {1}{p}}.} \)

Therefore, \( {\displaystyle \delta _{p}(\varepsilon )={\frac {1}{p2^{p}}}\varepsilon ^{p}+\cdots }. \)

See also

Uniformly smooth space

Notes

p. 60 in Lindenstrauss & Tzafriri (1979).
Day, Mahlon (1944), "Uniform convexity in factor and conjugate spaces", Ann. of Math., 2, Annals of Mathematics, 45 (2): 375–385, doi:10.2307/1969275, JSTOR 1969275
Lemma 1.e.8, p. 66 in Lindenstrauss & Tzafriri (1979).
see Remarks, p. 67 in Lindenstrauss & Tzafriri (1979).
see Proposition 1.e.6, p. 65 and Lemma 1.e.7, 1.e.8, p. 66 in Lindenstrauss & Tzafriri (1979).
see Pisier, Gilles (1975), "Martingales with values in uniformly convex spaces", Israel J. Math., 20 (3–4): 326–350, doi:10.1007/BF02760337, MR 0394135 .

Hanner, Olof (1955), "On the uniform convexity of \( L^{p} \) and \( \ell ^{p} \)", Arkiv för Mathematik, 3: 239–244, doi:10.1007/BF02589410

References

Beauzamy, Bernard (1985) [1982]. Introduction to Banach Spaces and their Geometry (Second revised ed.). North-Holland. ISBN 0-444-86416-4. MR 0889253.
Clarkson, James (1936), "Uniformly convex spaces", Trans. Amer. Math. Soc., American Mathematical Society, 40 (3): 396–414, doi:10.2307/1989630, JSTOR 1989630
Fuster, Enrique Llorens. Some moduli and constants related to metric fixed point theory. Handbook of metric fixed point theory, 133-175, Kluwer Acad. Publ., Dordrecht, 2001. MR1904276
Lindenstrauss, Joram and Benyamini, Yoav. Geometric nonlinear functional analysis Colloquium publications, 48. American Mathematical Society.
Lindenstrauss, Joram; Tzafriri, Lior (1979), Classical Banach spaces. II. Function spaces, Ergebnisse der Mathematik und ihrer Grenzgebiete [Results in Mathematics and Related Areas], 97, Berlin-New York: Springer-Verlag, pp. x+243, ISBN 3-540-08888-1.
Vitali D. Milman. Geometric theory of Banach spaces II. Geometry of the unit sphere. Uspechi Mat. Nauk, vol. 26, no. 6, 73-149, 1971; Russian Math. Surveys, v. 26 6, 80-159.

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