Very tricky problem from Spivak's Calculus, Ch. 4 & 5: Graphs/Limits

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The discussion focuses on the limits and behavior of the function defined by replacing all digits in the decimal expansion of a number after the first occurrence of 7 with zeros, as described in Spivak's Calculus, Chapters 4 and 5. The participants analyze the existence of limits for various values of 'a', particularly around points where the decimal expansion ends in 7 followed by repeating 9s, such as 0.7̅9. They conclude that the limit exists for all real numbers except those with a decimal expansion ending in 7̅9, where the left-hand limit equals 0.7 and the right-hand limit equals 0.8, indicating that the limit does not exist at these points.

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  • #31
PeroK said:
The main proof as you call it is potentially important, as it formalises the argument. But, without the side proof it means nothing.

That proposition must be false. The problem is that a small reduction in ##x## after a string of zeroes may cause a long string of nines. Using the double digit idea solves this problem.
The proposition should actually be with a ##\leq##, ie ##a-\delta \leq f(x)##. Is this what you mean?
 
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  • #32
zenterix said:
The proposition should actually be with a ##\leq##, ie ##a-\delta \leq f(x)##. Is this what you mean?
You're right. But, I don't see how that proposition helps.
 
  • #33
PeroK said:
This is effectively what you need to prove. Otherwise your ϵ−δ is just window-dressing!
It's the proof of this, which was a step in the main proof. It's required for the latter.
 

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