Finding the Right Delta: Epsilon-Delta Convergence in a Continuous Function

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SUMMARY

The discussion focuses on finding a suitable delta (δ) for the function f(x) = √x + 2x to demonstrate continuity based on the epsilon (ε)-delta definition. The user manipulates the expression |f(x) - f(y)| = |√x - √y| + 2|x - y| and establishes that by setting |x - y| < ε/4, the condition |f(x) - f(y)| < ε is satisfied. The conversation confirms that the approach is valid for proving continuity, although exploring the least upper bound for δ is deemed unnecessary.

PREREQUISITES
  • Understanding of epsilon-delta definitions of continuity
  • Familiarity with the properties of continuous functions
  • Knowledge of basic calculus, particularly limits and derivatives
  • Experience with manipulating inequalities in mathematical proofs
NEXT STEPS
  • Study the epsilon-delta definition of uniform continuity
  • Learn about the properties of continuous functions on closed intervals
  • Explore the concept of least upper bounds in real analysis
  • Investigate the implications of continuity in the context of differentiable functions
USEFUL FOR

Mathematics students, educators, and anyone interested in real analysis, particularly those studying continuity and its applications in calculus.

fyziky
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Hello all,
My question is as follows:
f:[1,[tex]\infty[/tex]) is defined by f(x)=[tex]\sqrt{x}[/tex]+2x (1[tex]\leq[/tex]x<[tex]\infty[/tex]) Given [tex]\epsilon[/tex]>0 find [tex]\delta[/tex]>0 such that if |x-y|<[tex]\delta[/tex] then |f(x)-f(y)|<[tex]\epsilon[/tex]

It seems I am being asked to show continuity, and not uniform continuity, so my approach is this, but I am not sure it works:
|f(x)-f(y)|=[tex]\sqrt{x}[/tex]-[tex]\sqrt{y}[/tex] + 2(x-y) after minor manipulation

since [tex]\sqrt{x}[/tex] function is uniformly continuous it is safe to set [tex]\sqrt{x}[/tex]-[tex]\sqrt{y}[/tex]<[tex]\delta[/tex]/2 and defining |x-y|<[tex]\delta[/tex]/4 we get

[tex]\sqrt{x}[/tex]-[tex]\sqrt{y}[/tex] + 2(x-y) [tex]\leq[/tex] [tex]\delta[/tex]/2 + 2[tex]\delta[/tex]/4=[tex]\delta[/tex]
since [tex]\delta[/tex] can equal [tex]\epsilon[/tex] we just need make |x-y|<[tex]\epsilon[/tex]/4

any help would be appreciated, I am not sure i have the right idea here. thank you.
 
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Yes, if you worded the problem correctly, then this will work because all it's asking is to find some [tex]\delta[/tex] that will work. However, just for kicks and giggles, see if you can find the least upper bound for [tex]\delta .[/tex]

EDIT: On second thought, there's really no need to go that extra step. Nothing really insightful to be gained. Forget it. Your answer's just fine.
 

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