Continuity of maps between function spaces and normed spaces

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Will the mapping ff of the space C[0;1]C[0 ; 1] (see problem 31) into itself be continuous, if it is given by the formula:

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(1)

f(x)=φxf(x) = \varphi x, where φ=φ(t)\varphi = \varphi (t) is a fixed function from C[0;1]C[0 ; 1], and x=x(t)∈C[0,1]x = x(t) \in C[0,1]

(2)

f(x)=x2,x(t)=C[0;1]f(x) = x^{2}, x(t) = C[0 ; 1]?

Will the mapping f(x)=x2f(x) = x^{2} of the space CL1[0;1]C L_{1}[0 ; 1] (see problem 3.5) into itself be continuous?

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Let X={x∈C1[0;1],x(0)=0}X = \left\{ x \in C^{1}[0 ; 1], x(0) = 0\right\} (see problem 93). Will the mapping f:X→C[0;1]f: X \rightarrow C[0 ; 1], given by the formulas f(x)(t)=x(t)t,t∈(0;1],f(x)(0)=lim⁡t→0x(t)tf(x)(t) = \frac{x(t)}{t}, t \in (0 ; 1], f(x)(0) = \lim_{t \rightarrow 0} \frac{x(t)}{t}, be continuous?

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Are the following mappings of the space C[0;1]C[0 ; 1] (see problem 31) into itself continuous:

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(1)

f(x)(t)=∫0tx(s)dsf(x)(t) = \int_{0}^{t} x(s) d s;

(2)

f(x)(t)=∫0tx2(s)ds;f(x)(t) = \int_{0}^{t} x^{2}(s) d s ;

(3)

f(x)(t)=∫0tsin⁡(t−s)x(s)ds,t∈[0;1]f(x)(t) = \int_{0}^{t} \sin (t-s) x(s) d s, t \in [0 ; 1]?

Are the following mappings ff of the space CL2[0;1]C L_{2}[0 ; 1] (see problem 3, part 8) into itself continuous:

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(1)

f(x)(t)=φ(t)x(t)f(x)(t) = \varphi (t) x(t), where φ(t)\varphi (t) is a fixed function continuous on the segment [0;1][0 ; 1];

(2)

f(x)(t)=x2(t)f(x)(t) = x^{2}(t);

(3)

f(x)(t)=∫0tx(s)ds,t∈[0;1]?f(x)(t) = \int_{0}^{t} x(s) d s, t \in [0 ; 1]?

Will the mapping f(x)=x(1)f(x) = x(1) be continuous, if it is considered as a mapping:

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(1)

of C[0;1]C[0 ; 1] (see problem 31) into RR;

(2)

of CL2[0;1]C L_{2}[0 ; 1] (see problem 3, part 8) into RR?

Is differentiation, regarded as a mapping of the subspace consisting of continuously differentiable functions of the space C[a;b]C[a ; b] (see problem 31) into the space C[a;b]C[a ; b] itself, continuous?

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Is differentiation, regarded as a mapping of the space C1[a;b]C^{1}[a ; b] (see problem 93) onto the space C[a;b]C[a ; b] (see problem 31), continuous?

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Let F:C[0,1]→RF: C[0,1] \rightarrow \mathbb {R} be defined by F(f)=f(0)F(f)=f(0), and let g(x)=x2g(x)=x^{2}. Find lim⁡f→gF(f)\lim_{f \rightarrow g} F(f).

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Let dd be a metric on C[0,1]C[0,1] defined by

d(f,g)=∫01∣f(x)−g(x)∣dx d(f, g)=\int _{0}^{1}\left|f(x)-g(x)\right| d x

(see Exercise 2.1.3). Let F:C[0,1]→RF: C[0,1] \rightarrow \mathbb {R} be defined by F(f)=f(0)F(f)=f(0), and let g(x)=x2g(x)=x^{2}. Find lim⁡f→gF(f)\lim_{f \rightarrow g} F(f).

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Let dd be a metric on R\mathbb {R} defined by

d(x,y):=∣x−y∣1+∣x−y∣ d(x, y):=\frac{\left|x-y\right|}{1+\left|x-y\right|}

(see Exercise 2.1.8), and let ρ\rho denote the usual distance on R,ρ(x,y)=∣x−y∣\mathbb {R}, \rho (x, y)=\left|x-y\right|. Let f:(R,d)→(R,ρ)f:(\mathbb {R}, d) \rightarrow (\mathbb {R}, \rho ) be defined by f(x)=x2f(x)=x^{2}. Find lim⁡x→2f(x)\lim_{x \rightarrow 2} f(x).

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For a normed linear space XX, show that the function ∥⋅∥:X→R\left\| \cdot \right\| : X \rightarrow \mathbb {R} is continuous.

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Let XX be a normed linear space.

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(i)

Let {xn}\left\{ x_{n}\right\} and {yn}\left\{ y_{n}\right\} be sequences in XX such that {xn}→x\left\{ x_{n}\right\} \rightarrow x and {yn}→y\left\{ y_{n}\right\} \rightarrow y. Show that for any real numbers α\alpha and β,{αxn+βyn}→αx+βy\beta ,\left\{ \alpha x_{n}+\beta y_{n}\right\} \rightarrow \alpha x+\beta y.

(ii)

Use (i) to show that if YY is a subspace of XX, then its closure Yˉ\bar{Y} also is a linear subspace of XX.

(iii)

Use (i) to show that the vector sum is continuous from X×XX \times X to XX and scalar multiplication is continuous from R×X\mathbb {R} \times X to XX.

Verify that the functions Ti,i=1,2,3T_{i}, i = 1,2,3 defined by (4.42) are continuous on C[0,1]\mathcal{C}[0,1].

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References

[ККЧ+03] Кудрявцев Л.Д., Кутасов А.Д., Чехлов В.И., Шабунин М.И. Функции многих переменных. 2-е изд., ФИЗМАТЛИТ, 2003.
[Pet25] Petrovic J.S. Measure and Integral: Theory and Practice. CRC Press, 2025.
[RoFi23] Royden H.L., Fitzpatrick P.M. Real Analysis. 5th ed., Pearson, 2023.
[AtLa06] Athreya K.B., Lahiri S.N. Measure Theory and Probability Theory. Springer, 2006.