We denote by $K$ the function defined from $[0,1]^2$ to $\mathbb{R}$ by the following relation: $K(s,t) = (1-s)t$ if $0 \leq t \leq s \leq 1$ and $K(s,t) = (1-t)s$ otherwise. We denote by $T$ the application defined on $E = C([0,1], \mathbb{R})$, equipped with the norm $\|.\|_2 = \sqrt{\int_0^1 |f(x)|^2 \, dx}$, by the relation: $$\forall f \in E, \quad \forall s \in [0,1], \quad T(f)(s) = \int_0^1 K(s,t) f(t) \, dt$$ Show that if $\lambda \in \sigma_p(T)$ and $f \in \operatorname{Ker}(T - \lambda Id)$, then $f \in C^2([0,1], \mathbb{R})$ and satisfies the equation $$\lambda f'' + f = 0$$ with the conditions $f(0) = f(1) = 0$.

We denote by $K$ the function defined from $[0,1]^2$ to $\mathbb{R}$ by the following relation: $K(s,t) = (1-s)t$ if $0 \leq t \leq s \leq 1$ and $K(s,t) = (1-t)s$ otherwise. We denote by $T$ the application defined on $E = C([0,1], \mathbb{R})$, equipped with the norm $\|.\|_2 = \sqrt{\int_0^1 |f(x)|^2 \, dx}$, by the relation: $$\forall f \in E, \quad \forall s \in [0,1], \quad T(f)(s) = \int_0^1 K(s,t) f(t) \, dt$$ Deduce $\sigma_p(T)$. Calculate the eigenspaces $E_{\lambda} = \operatorname{Ker}(T - \lambda Id)$ associated with each element $\lambda \in \sigma_p(T)$.

Let $\lambda \in \mathbb { R }$. Show that the problem
$$\left\{ \begin{array} { l } - v _ { \lambda } ^ { \prime \prime } ( x ) + c ( x ) v _ { \lambda } ( x ) = f ( x ) , x \in [ 0,1 ] \\ v _ { \lambda } ( 0 ) = 0 \\ v _ { \lambda } ^ { \prime } ( 0 ) = \lambda \end{array} \right.$$
admits a unique solution $v _ { \lambda } \in \mathcal { C } ^ { 2 } ( [ 0,1 ] , \mathbb { R } )$.

Show that for all $\lambda \in \mathbb { R } , v _ { \lambda }$ can be expressed in the form:
$$v _ { \lambda } = \lambda w _ { 1 } + w _ { 2 }$$
with $w _ { 1 } \in \mathcal { C } ^ { 2 } ( [ 0,1 ] , \mathbb { R } )$ the unique solution of the system
$$\left\{ \begin{array} { l } - w _ { 1 } ^ { \prime \prime } ( x ) + c ( x ) w _ { 1 } ( x ) = 0 , x \in [ 0,1 ] \\ w _ { 1 } ( 0 ) = 0 \\ w _ { 1 } ^ { \prime } ( 0 ) = 1 \end{array} \right.$$
and $w _ { 2 }$ a function independent of $\lambda$ to be characterized.

Show that $w _ { 1 } ( 1 ) \neq 0$.

Deduce that there exists a solution $u \in \mathcal { C } ^ { 2 } ( [ 0,1 ] , \mathbb { R } )$ to problem (1): $$\left\{ \begin{array} { l } - u ^ { \prime \prime } ( x ) + c ( x ) u ( x ) = f ( x ) , x \in [ 0,1 ] \\ u ( 0 ) = u ( 1 ) = 0 \end{array} \right.$$ Show that this solution is unique.

We now assume $\lambda \neq 0$. Give a necessary and sufficient condition for the differential equation (II.2) $$z''(\theta) + \lambda z(\theta) = 0$$ to admit non-zero $2\pi$-periodic solutions. Give these solutions.

In this subsection II.C, we consider two functions of class $\mathcal{C}^2$, $u : \mathbb{R}^{*+} \rightarrow \mathbb{R}$ and $v : \mathbb{R} \rightarrow \mathbb{R}$ and we set $$\forall (r,\theta) \in \mathbb{R}^{*+} \times \mathbb{R} \quad f(r\cos(\theta), r\sin(\theta)) = u(r)v(\theta)$$ We now assume $\lambda \neq 0$. Give a necessary and sufficient condition for (II.2) $$z''(\theta) + \lambda z(\theta) = 0$$ to admit non-zero $2\pi$-periodic solutions. Give these solutions.

For all $f \in E$, we set, $$\forall s \in [0,1], \quad T(f)(s) = \int_0^1 k_s(t) f(t)\,\mathrm{d}t$$ Let $\lambda \in \mathbb{R}$ be a nonzero eigenvalue of $T$ and $f$ be an associated eigenvector. Show that $f$ is a solution of the differential equation $\lambda f'' = -f$.

In this part, $E$ denotes the vector space of functions $f : [0,1] \rightarrow \mathbb{R}$ continuous, equipped with the inner product defined by, $$\forall (f,g) \in E^2, \quad \langle f, g \rangle = \int_0^1 f(t) g(t) \, \mathrm{d}t$$ For all $s \in [0,1]$, we define the function $k_s$ by, $$\forall t \in [0,1], \quad k_s(t) = \begin{cases} t(1-s) & \text{if } t < s \\ s(1-t) & \text{if } t \geqslant s. \end{cases}$$ For all $f \in E$, we set, $$\forall s \in [0,1], \quad T(f)(s) = \int_0^1 k_s(t) f(t) \, \mathrm{d}t$$ Let $\lambda \in \mathbb{R}$ be a nonzero eigenvalue of $T$ and $f$ be an associated eigenvector. Show that $f$ is a solution of the differential equation $\lambda f'' = -f$.

Let $T > 0$ be a real number and $n \in \mathbb{N}^*$ be a natural integer. Let $A : \mathbb{R} \rightarrow \mathscr{M}_n(\mathbb{C})$ be an application continuous on $\mathbb{R}$ and $T$-periodic. Let $B \in \mathscr{M}_n(\mathbb{C})$ be the matrix such that $M(t+T) = M(t)\exp(TB)$ for all $t$. Consider the differential system $$X'(t) = A(t) X(t) + b(t) \tag{3}$$ where $b : \mathbb{R} \rightarrow \mathbb{C}^n$ is a continuous function on $\mathbb{R}$ and $T$-periodic. We assume that $1$ is not an eigenvalue of $\exp(TB)$. Show that (3) possesses a unique $T$-periodic solution.

Justify that there exists a unique solution $u$ to the Cauchy problem $\left( C _ { \ell } \right)$, give its expression and draw its variation table.
$$\left( C _ { \ell } \right) : \left\{ \begin{array} { l } u ^ { \prime } ( x ) + u ( x ) + 1 = \frac { 1 } { 2 } ( 1 + u ( x ) ) \\ u ( 0 ) = 0 \end{array} . \right.$$

The solution of the differential equation $\frac { d y } { d x } + \frac { y } { 2 } \sec x = \frac { \tan x } { 2 y }$, where $0 \leq x < \frac { \pi } { 2 }$ and $y ( 0 ) = 1$, is given by
(1) $y ^ { 2 } = 1 + \frac { x } { \sec x + \tan x }$
(2) $y = 1 + \frac { x } { \sec x + \tan x }$
(3) $y = 1 - \frac { x } { \sec x + \tan x }$
(4) $y ^ { 2 } = 1 - \frac { x } { \sec x + \tan x }$