For $i \in \mathbf{Z}$, we define $v_i$ in $\mathbf{C}^{\mathbf{Z}}$ by $$v_i(k) = \left\{\begin{array}{l} 1 \text{ if } k = i \\ 0 \text{ if } k \neq i \end{array}\right.$$
3a. Show that the family $\{v_i\}_{i \in \mathbf{Z}}$ is a basis of $V$.
3b. Calculate $E(v_i)$.

Let $\lambda, \mu \in \mathbf{C}^{\mathbf{Z}}$. We define the linear maps $F, H \in \mathcal{L}(V)$ respectively by $$H(v_i) = \lambda(i) v_i \quad \text{and} \quad F(v_i) = \mu(i) v_{i+1}, \quad i \in \mathbf{Z}.$$ Show that $H \circ E = E \circ H + 2E$ if and only if for all $i \in \mathbf{Z}, \lambda(i) = \lambda(0) - 2i$.

We assume that the conditions of question 4 are satisfied (i.e., $\lambda(i) = \lambda(0) - 2i$ for all $i \in \mathbf{Z}$). Let $\lambda, \mu \in \mathbf{C}^{\mathbf{Z}}$ and $F, H \in \mathcal{L}(V)$ defined by $H(v_i) = \lambda(i) v_i$ and $F(v_i) = \mu(i) v_{i+1}$. Show that $E \circ F = F \circ E + H$ if and only if for all $i \in \mathbf{Z}$, $$\mu(i) = \mu(0) + i(\lambda(0) - 1) - i^2.$$

Show that the set $E^{c}$ is non-empty.

Is the set $E^{c}$ a vector subspace of $\mathbb{R}^{\mathbb{N}}$?

Show that $E^{c}$ is strictly included in $E$.

We have $D_n = \left\{\sum_{j=1}^{n} \frac{x_j}{2^j}, (x_j)_{j \in \llbracket 1,n \rrbracket} \in \{0,1\}^n\right\}$ and $D = \bigcup_{n \in \mathbb{N}^{\star}} D_n$.
Establish the monotonicity in the sense of inclusion of the sequence $(D_n)_{n \geqslant 1}$ and then verify $D \subset [0,1[$.

We have $d_{n+1}(x) = 2^{n+1}(\pi_{n+1}(x) - \pi_n(x))$ for all $(x,n) \in \mathbb{R} \times \mathbb{N}$.
Establish $$\forall (x,j) \in \mathbb{R} \times \mathbb{N}^{\star}, \quad d_j(x) \in \{0,1\}.$$

We have $D_n = \left\{\sum_{j=1}^{n} \frac{x_j}{2^j}, (x_j)_{j \in \llbracket 1,n \rrbracket} \in \{0,1\}^n\right\}$ and $\pi_n(x) = \frac{\lfloor 2^n x \rfloor}{2^n}$.
Let $n \in \mathbb{N}^{\star}$. Justify $x \in D_n \Longleftrightarrow 2^n x \in \llbracket 0, 2^n - 1 \rrbracket$.

Let $n \in \mathbb{N}^{\star}$. Show that the application $$\Psi_n : \begin{gathered} \{0,1\}^n \rightarrow D_n \\ (x_j)_{j \in \llbracket 1,n \rrbracket} \mapsto \sum_{j=1}^{n} \frac{x_j}{2^j} \end{gathered}$$ is bijective.

For all $(x,n) \in \mathbb{R} \times \mathbb{N}$, we define $\pi_n(x) = \frac{\lfloor 2^n x \rfloor}{2^n}$ and $d_{n+1}(x) = 2^{n+1}(\pi_{n+1}(x) - \pi_n(x))$.
Establish $$\forall (x,j) \in \mathbb{R} \times \mathbb{N}^{\star}, \quad d_j(x) \in \{0,1\}.$$

Let $k$ be a non-negative integer. We define the function $f _ { k }$ from the interval $[ - 1,1 ]$ to itself by $f _ { k } ( x ) = \cos ( k \arccos x )$.
a) Expand the expression $f _ { k + 1 } ( x ) + f _ { k - 1 } ( x )$, and deduce the relation $$\forall x \in [ - 1,1 ] \quad f _ { k + 1 } ( x ) = 2 x f _ { k } ( x ) - f _ { k - 1 } ( x )$$
b) Deduce that $f _ { k }$ identifies on $[ - 1,1 ]$ with a polynomial $T _ { k }$, of degree $k$, with the same parity as $k$.

We consider a natural integer $n$ and a complex number $a$. We define a family of polynomials $(A_0, A_1, \ldots, A_n)$ by setting $$A_0 = 1 \quad \text{and, for all } k \in \llbracket 1, n \rrbracket, \quad A_k = \frac{1}{k!} X(X - ka)^{k-1}.$$ Using the result $A_k'(X) = A_{k-1}(X-a)$, deduce, for $j$ and $k$ elements of $\llbracket 0, n \rrbracket$, the value of $A_k^{(j)}(ja)$. Distinguish according to whether $j < k$, $j = k$ or $j > k$.

For $n \in \mathbb{N}$, calculate $\left\|T_n\right\|_{L^\infty([-1,1])}$.
The sequence of polynomials $\left(T_n\right)_{n \in \mathbb{N}}$ is defined by $T_0 = 1, T_1 = X$ and $\forall n \in \mathbb{N}, T_{n+2} = 2X T_{n+1} - T_n$.

For $n \in \mathbb{N}$, calculate $\left\|T_n\right\|_{L^\infty([-1,1])}$.
The sequence of polynomials $\left(T_n\right)_{n \in \mathbb{N}}$ is defined by $T_0 = 1, T_1 = X$ and $\forall n \in \mathbb{N}, T_{n+2} = 2X T_{n+1} - T_n$.

Let $p \in \mathbb{N}$. Show that the sequence with general term $u_n = \binom{n}{p}$ is hypergeometric.

Prove that the set of sequences satisfying relation $$P(n) u_n = Q(n) u_{n+1}$$ with $$P(X) = X(X-1)(X-2) \quad \text{and} \quad Q(X) = X(X-2)$$ is a vector space for which we will specify a basis and the dimension.

The Pochhammer symbol is defined, for any real number $a$ and any natural integer $n$, by $$[a]_n = \begin{cases} 1 & \text{if } n = 0 \\ a(a+1)\cdots(a+n-1) = \prod_{k=0}^{n-1}(a+k) & \text{otherwise} \end{cases}$$ If $a$ is a negative or zero integer, justify that the sequence $\left([a]_n\right)_{n \in \mathbb{N}}$ is zero from a certain rank onwards.

The Pochhammer symbol is defined, for any real number $a$ and any natural integer $n$, by $$[a]_n = \begin{cases} 1 & \text{if } n = 0 \\ a(a+1)\cdots(a+n-1) = \prod_{k=0}^{n-1}(a+k) & \text{otherwise} \end{cases}$$ Let $a \in \mathbb{R}$. Verify that, for any natural integer $n$, $[a]_{n+1} = a[a+1]_n$.

Given three real numbers $a, b$ and $c$, the Gauss hypergeometric function associated with the triplet $(a, b, c)$ is defined by $$F_{a,b,c}(x) = \sum_{n=0}^{+\infty} \frac{[a]_n [b]_n}{[c]_n} \frac{x^n}{n!}$$ Justify that, if $c \in D$, then $\frac{[a]_n [b]_n}{[c]_n}$ is well defined for any natural integer $n$.

Using question 4, show $$\forall n \in \mathbb { N } , \quad C _ { n + 1 } = \sum _ { r = 0 } ^ { n } C _ { r } C _ { n - r } .$$

Let $\left( U _ { n } \right) _ { n \in \mathbb { N } }$ be the sequence of polynomials defined by $U _ { 0 } = 1 , U _ { 1 } = X - 1$ and $\forall n \in \mathbb { N } , U _ { n + 2 } = ( X - 2 ) U _ { n + 1 } - U _ { n }$. For all $n \in \mathbb { N }$, show that $U _ { n }$ is monic of degree $n$, and determine the value of $U _ { n } ( 0 )$.

Let $\left( U _ { n } \right) _ { n \in \mathbb { N } }$ be the sequence of polynomials defined by $U _ { 0 } = 1 , U _ { 1 } = X - 1$ and $\forall n \in \mathbb { N } , U _ { n + 2 } = ( X - 2 ) U _ { n + 1 } - U _ { n }$. Let $\theta \in \mathbb { R }$. Show that $\forall n \in \mathbb { N } , U _ { n } \left( 4 \cos ^ { 2 } \theta \right) \sin \theta = \sin ( ( 2 n + 1 ) \theta )$.

In this subsection, $I = ]{-1,1}[$ and $w(x) = \frac{1}{\sqrt{1-x^2}}$. For every integer $n \in \mathbb{N}$, consider the function $Q_n : \left|\,\begin{array}{ccl} [-1,1] & \rightarrow & \mathbb{R} \\ x & \mapsto & \cos(n \arccos(x)) \end{array}\right.$.
Calculate $Q_0$, $Q_1$ and, for all $n \in \mathbb{N}$, express simply $Q_{n+2}$ in terms of $Q_{n+1}$ and $Q_n$.

In this subsection, $I = ]{-1,1}[$ and $w(x) = \frac{1}{\sqrt{1-x^2}}$. For every integer $n \in \mathbb{N}$, consider the function $Q_n(x) = \cos(n \arccos(x))$ on $[-1,1]$.
Deduce that, for all $n \in \mathbb{N}$, $Q_n$ is polynomial and determine its degree and leading coefficient.