Is such a theorem exists? (Uniform convergence).

Click For Summary
SUMMARY

The discussion centers on the existence of a theorem relating to uniform continuity for functions of multiple variables. The user proposes that if a function f(x1,...,xn) is continuous and differentiable with bounded partial derivatives, then it should be uniformly continuous. However, a counter-example is presented with the function f_n(x) = x^n, which suggests that this proposition may not hold true. The conversation also touches on the implications of bounded partial derivatives on the total derivative and directional derivatives, indicating that boundedness in these derivatives restricts the function's rate of change.

PREREQUISITES
  • Understanding of uniform continuity in multivariable calculus
  • Knowledge of differentiable functions and their properties
  • Familiarity with partial derivatives and total derivatives
  • Basic concepts of counter-examples in mathematical proofs
NEXT STEPS
  • Research the implications of bounded derivatives on uniform continuity
  • Study the properties of the function f_n(x) = x^n and its behavior as n approaches infinity
  • Explore the relationship between directional derivatives and total derivatives in multivariable calculus
  • Investigate existing theorems related to uniform continuity in higher dimensions
USEFUL FOR

Mathematicians, calculus students, and educators interested in the properties of multivariable functions and the nuances of uniform continuity.

MathematicalPhysicist
Science Advisor
Gold Member
Messages
4,662
Reaction score
372
I would like to know if there's a counterpart to the single variable theorem, that if f is a differentialble function with a bounded derivative, is uniformly continuous.

I think the counterpart should be, if f(x1,...,xn) is continuous function, and differentiable, and each f'_xi are bounded then f is uniformly continuous.

But I have my suspicions.

Anyone can corroborate or disprove this?
 
Physics news on Phys.org
I think f_n:[0,1]-->R defined by f_n(x)=x^n is a counter-example.
 
quasar987 said:
I think f_n:[0,1]-->R defined by f_n(x)=x^n is a counter-example.

I'm not sure what a sequence of functions has to do with anything here...

If each of the partial derivatives are bounded, the total derivative (which is the matrix of every partial derivative) has each entry bounded. The directional derivative, which tells you how fast the function changes when you go in a certain direction, is equal to the total derivative matrix multiplied by a (unit) vector in the direction you want to look at. So this is bounded also, which means that the function cannot increase faster than a certain value (if the partial derivatives are all bounded by M, then this is M*n where n is the dimension of your domain. If your image space is more than one dimension, say dimension k, it's M*n*k)
 

Similar threads

  • · Replies 1 ·
Replies
1
Views
3K
  • · Replies 3 ·
Replies
3
Views
3K
  • · Replies 2 ·
Replies
2
Views
3K
  • · Replies 1 ·
Replies
1
Views
3K
  • · Replies 9 ·
Replies
9
Views
3K
  • · Replies 3 ·
Replies
3
Views
4K
  • · Replies 1 ·
Replies
1
Views
3K
  • · Replies 22 ·
Replies
22
Views
3K
  • · Replies 4 ·
Replies
4
Views
7K
  • · Replies 2 ·
Replies
2
Views
3K