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farleyknight
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Suppose you had some arbitrary function f : R^n \to R^p and x \in R^n. You want to know if it's continuous, so you do some epsilon-delta to find out for sure. However, only the most simple functions permit this without some extra restrictions.
Consider f(x) = x^2. To show that |x - a| < \delta \rightarrow |f(x) - f(a)| < \epsilon, you'd have to break up |x^2 - a^2| into |x + a||x - a|. The next step is something I'm somewhat concerned about, but not even for this particular function. The technique I want to illustrate ought to work for higher powers. If it's a correct technique, it ought to work for the general case.
In general, proofs of continuity for anything but linear combinations would require that you make an assumption |x - a| < 1 and then |x| < 1 + |a|, by the triangle inequality. In this case, we took the witnessing constant 1 to create the coefficient (|1 + |a| + a|) to be sure that this neighborhood (|1 + |a| + a|)|x - a| < \epsilon holds for the delta we wish to find.
Then when you get around to finding epsilon, you take delta as the min of whatever epsilon and 1, or \delta = \min( 1, F(\epsilon) ) where F(\epsilon) is a function of your constant a and any witnessing constants.
My question is: Will this always work? Can I just introduce witnessing constants to replace x whenever I don't want it? (Provided I maintain the inequality.) Will you ever run into trouble with this as a general strategy? I've seen this in all the books I've come across, so I feel the answer is yes, but having someone more experienced will help solidify that.
Surely I will need more advanced techniques for higher level courses, but is there anything I should watch for at my current level (intro analysis course)?
Consider f(x) = x^2. To show that |x - a| < \delta \rightarrow |f(x) - f(a)| < \epsilon, you'd have to break up |x^2 - a^2| into |x + a||x - a|. The next step is something I'm somewhat concerned about, but not even for this particular function. The technique I want to illustrate ought to work for higher powers. If it's a correct technique, it ought to work for the general case.
In general, proofs of continuity for anything but linear combinations would require that you make an assumption |x - a| < 1 and then |x| < 1 + |a|, by the triangle inequality. In this case, we took the witnessing constant 1 to create the coefficient (|1 + |a| + a|) to be sure that this neighborhood (|1 + |a| + a|)|x - a| < \epsilon holds for the delta we wish to find.
Then when you get around to finding epsilon, you take delta as the min of whatever epsilon and 1, or \delta = \min( 1, F(\epsilon) ) where F(\epsilon) is a function of your constant a and any witnessing constants.
My question is: Will this always work? Can I just introduce witnessing constants to replace x whenever I don't want it? (Provided I maintain the inequality.) Will you ever run into trouble with this as a general strategy? I've seen this in all the books I've come across, so I feel the answer is yes, but having someone more experienced will help solidify that.
Surely I will need more advanced techniques for higher level courses, but is there anything I should watch for at my current level (intro analysis course)?
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