I have been stuck on this Real Analysis problem for hours and am just totally clueless- I am sure it

Annabel Sullivan

Annabel Sullivan

Answered question

2022-05-08

I have been stuck on this Real Analysis problem for hours and am just totally clueless- I am sure it is some application of the Intermediate Value Theorem- suppose   f : R R is continuous at every point. Prove that the equation   f ( x ) = c cannot have exactly two solutions for every value of   c .

Would appreciate some help

Answer & Explanation

Percyaehyq

Percyaehyq

Beginner2022-05-09Added 18 answers

If every value of f occurs exactly once, there is nothing to prove.

Otherwise, let c be such that f ( a ) = f ( b ) = c for a < b.

If f is constant in [ a, b], then we are done, because c is attained at least 3 times.

Otherwise, f attains its maximum M at an interior point u in [ a, b]. The value M is then a local maximum. (This fails if M= c, but in this case take the minimum instead.)

If f does not attain the value M at another point in R , then we are done.

Otherwise, let v u be such that f ( v ) = M ( v is not necessarily in [ a, b]).

For small enough ϵ > 0, the value M ε is then attained at least 3 times: twice near u and at least once near v.

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