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A SUFFICIENT CONDITION FOR A FUNCTION TO SATISFY A WEAK LIPSCHITZ CONDITION

RADU MICULESCU

For a function f : X Y let (X, d) and (Y, ρ) be metric spaces consider the following condition: for eachx X there existsCx (0,1) such that for each sequence S = (xn)n∈N X satisfying d(xn, x0) Cxn, n N, there exists MS Rsuch that ρ(f(xn), f(x0))MSCxn, n N. We show that the above condition is sufficient forfto be Lipschitz at each point ofX.

AMS 2000 Subject Classification: 54C08, 26A16.

Key words: Lipschitz function at a point.

1. INTRODUCTION

Let us start with a basic definition.

Definition. Let (X, d) and (Y, ρ) be metric spaces. A functionf :X→Y is called Lipschitz if there exists a constantM such that

ρ(f(x), f(y))≤M·d(x, y) for allx, y∈X.

From the point of view of mathematical analysis, the condition of being Lipschitz may be considered as a weakened version of differentiability, on ac- count of a famous result of Rademacher (see [2]), which states that a Lipschitz functionf : U = U Rn Rm is differentiable outside of a Lebesgue null subset ofU.

From the point of view of mechanics, a Lipschitz map is one that obeys speed limits.

Lipschitz functions are used in the study of differential equations, mea- sure theory, functional analysis, and in the construction of mathematical mod- els in mechanics, physics and economy.

Lipschitz functions have been intensively studied. Two basic references in the area of Lipschitz functions are [1] and [3].

Therefore, it is important to recognize the Lipschitz functions.

A weak version of the Lipschitz condition is given by the following

MATH. REPORTS9(59),3 (2007), 275–278

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276 Radu Miculescu 2

Definition. Let (X, d) and (Y, ρ) be metric spaces andx∈X. A function f :X →Y is called Lipschitz at x if there exists a neighborhood U of x and a constantM such that

ρ(f(y), f(x))≤M d(y, x) for ally∈U.

Clearly, if we consider a functionf :X →Y, which is Lipschitz at a point x∈X, then for each C∈(0,1) and each sequence (xn)n∈N ⊆U satisfying

d(xn, x)≤Cn, n∈N, we have

ρ(f(xn), f(x))≤M Cn, n∈N.

Therefore, it is natural to look for sufficient conditions (in terms of the above discussion) for a function to be Lipschitz at the points of its domain.

In this paper we present a sufficient condition for a function to be Lips- chitz at each point of its domain.

2. THE RESULT

Theorem. Let (X, d) and (Y, ρ) be two metric spaces. Let f :X →Y be a function with the following property: for each x X there exists Cx (0,1) such that for each sequence S = (xn)n∈N ⊆X satisfying the condition d(xn, x0) Cxn, n N, there exists MS R such that ρ(f(xn), f(x0)) MSCxn,n∈N. Then, f is Lipschitz at any x∈X.

Proof. We shall show that for every x X there exist Mx R and δxRsuch that

y∈B(x, δx)⇒ρ(f(y), f(x))≤Mxd(y, x).

Let us suppose, for a contradiction that this is not the case. Therefore, there exists x0 X such that for all M R+ and δ R+, there exists xM,δ ∈X such that

d(xM,δ, x0)< δ and

ρ(f(xM,δ), f(x0))> Ld(xM,δ, x0).

In particular, choosing M = m N and δ = Cxn0, n N, we get xn,m∈X such that

(1) d(xn,m, x0)< Cxn0 and

(2) ρ(f(xn,m), f(x0))> md(xn,m, x0), for alln, m∈N.

(3)

3 A sufficient condition for a function to satisfy a weak Lipschitz condition 277

Inequality (2) implies

d(xn,m, x0)>0, n, m∈N. Since lim

n→∞Cxn0 = 0, we can define

n1 = max{n∈N |0< d(x1,1, x0)≤Cxn0}, n2 = max{n∈N |0< d(xn1+1,2, x0)≤Cxn0}, n3 = max{nN |0< d(xn2+1,3, x0)≤Cxn0} and, ifnk has been defined, then

nk+1= max{n∈N |0< d(xnk+1,k+1, x0)≤Cxn0}. Let us note that from (1) we have

d(xnk+1,k+1, x0)< Cxn0k+1, so that

nk+ 1∈ {n∈N |0< d(xnk+1,k+1, x0)≤Cxn0}, hencenk+ 1≤nk+1, i.e. nk< nk+1, for all k∈N.

Therefore, the sequence (nk)k∈N is strictly increasing.

Let us consider the sequence (un)n∈N defined as x1,1, . . . , x1,1

n1times , xn1+1,2, . . . , xn1+1,2

n2−n1times , . . . . By the definition ofnk, for allk∈N, we have (3) Cxn0k+1< d(xnk−1+1,k, x0)≤Cxn0k.

On the one hand, the sequence (d(un, x0))n∈N is in fact d(x1,1, x0), . . . , d(x1,1

n1times , x0), d(xn1+1,2, x0), . . . , d(xn1+1,2

n2−n1 times , x0), . . . .. Since by (3) we have

d(x1,1, x0)≤Cxn01 ≤Cxn01−1 ≤ · · · ≤Cx10, d(xn1+1,2, x0)≤Cxn02 ≤Cxn02−1 ≤ · · · ≤Cxn01+1,

· · ·

d(xnk−1+1,k, x0)≤Cxn0k ≤Cxn0k−1 ≤ · · · ≤Cxn0k−1+1,

· · · we getd(un, x0)≤Cxn0, for all n∈N.

On the other hand, the sequence (ρ(f(unk), f(x0))n∈N is

ρ(f(x1,1), f(x0)), ρ(f(xn1+1,2), f(x0)), . . . , ρ(f(xnk−1+1,k), f(x0)), . . . . From (2) we have

ρ(f(xnk−1+1,k), f(x0))≥kd(xnk−1+1,k, x0),

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278 Radu Miculescu 4

so, by (3), we obtain

ρ(f(xnk−1+1,k), f(x0))≥kCxn0k+1 =kCx0Cxn0k, i.e.,ρ(f(unk), f(x0)≥kCx0Cxn0k, k∈N.

According to the hypothesis, for the sequenceS0 = (un)n∈N there exists MS0 such thatρ(f(un), f(x0))≤MS0Cxn0 for alln∈N. Then fork∈N such thatkCx0 > MS0 we get the contradiction

ρ(f(unk), f(x0))≤MS0Cxn0k < kCx0Cxn0k ≤ρ(f(unk), f(x0).

Therefore, for each x X there exists a neighborhood U of x and a constant M such that

ρ(f(y), f(x))≤M d(y, x) for ally∈U, i.e. f is Lipschitz at x.

REFERENCES

[1] J. Luukkainen and J. V¨ais¨al¨a,Elements of Lipschitz topology. Ann. Acad. Sci. Fenn. Ser.

A I Math.3(1977),1, 85–122.

[2] H. Rademacher,Uber partielle und totale Differenzierbarkeit von Functionen mehrerer Variabeln und uber die Transformation der Doppelintegrale. Math. Ann. 79 (1919), 340–359.

[3] N. Weaver,Lipschitz Algebras. World Scientific Publishing Co., 1999.

Received 31 January 2006 University of Bucharest

Faculty of Mathematics and Computer Science Str. Academiei 14

010014 Bucharest, Romania miculesc@yahoo.com

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