Examples on boundedness, range and fixed points of functions

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Let Bˉ′(0,1)\bar{B}^{\prime }(0,1) be the punctured closed unit ball of the space Rn\mathbb {R}^{n} (i.e., the ball with the center removed). Give an example of a continuous mapping of this set into itself having no fixed point.

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Let m[f]m[f] and M[f]M[f] denote respectively the infimum and supremum of the function f(x)f(x) on the interval (a,b)(a,b).

Prove that if f1(x)f_{1}(x) and f2(x)f_{2}(x) are functions defined on (a,b)(a,b), then

m[f1+f2]⩾m[f1]+m[f2] m[f_{1}+f_{2}] \geqslant m[f_{1}]+m[f_{2}]

and

M[f1+f2]⩽M[f1]+M[f2]. M[f_{1}+f_{2}] \leqslant M[f_{1}]+M[f_{2}] .

Give examples of functions f1(x)f_{1}(x) and f2(x)f_{2}(x) such that in the last two relations: (a) equality holds, (б) strict inequality holds.

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(a)
(b)

Give an example of a function continuous on an interval:

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(1)

unbounded on this interval;

(2)

bounded on this interval, but attaining neither its supremum nor its infimum.

Give an example of a discontinuous function, defined on a closed interval, whose range is a closed interval.

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Give an example of a continuous function that takes the values 1 and 3, but does not take the value 2.

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Give an example of a function, continuous on an interval, whose range is:

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(1)

a closed interval;

(2)

an open interval;

(3)

a half-open interval.

Give an example of a function:

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(1)

continuous on the interval (0;1)(0 ; 1);

(2)

continuous on R\mathbb {R}, for which the equation f(x)=xf(x) = x : has no solutions.

Give an example of a function defined on a closed interval and unbounded on it.

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For each of the following sets give an example of a continuous mapping of the set into itself that has no fixed point:

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(a)

R\mathbb {R}.

(b)

(0,1](0,1].

(c)

[−10,−7]∪[2,4][-10,-7] \cup [2,4].

Give an example of a mapping of [0,1][0,1] into itself that is not continuous and has no fixed points.

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References

[БСШ17] Бородин П.А., Савчук А.М., Шейпак И.А. Задачи по функциональному анализу. МЦНМО, 2017.
[Dem97] Demidovich B.P. Сборник задач и упражнений по математическому анализу. 13-е изд., Издательство Московского университета / Издательство "ЧеРо", 1997.
[ККЧ+03] Кудрявцев Л.Д., Кутасов А.Д., Чехлов В.И., Шабунин М.И. Предел. Непрерывность. Дифференцируемость. 2-е изд., ФИЗМАТЛИТ, 2003.
[Sax02] Saxe K. Beginning Functional Analysis. Springer, 2002.