See here.

Regards M.I.

Please, I have installed Mathematica 12.1, it works now, please I need to Plot the solution, it doesn't work for me, why?

Grid[Partition[
Table[Plot[Evaluate[u[x, t] /. sol[[1]]], {x, 0, 1}, Exclusions -> None, PlotRange -> All], {t, 0, 12}], 3], ItemSize -> 10]

Please, Mariusz one last question: I modified the initial data as follows:

    t1 = 2;
    indicator[x_] := Piecewise[{{1, 0 < x < 1/2}}, 0];
(* problème du controle*)
E1 = D[u[x, t], {x, 2}] - D[u[x, t], {t, 2}] == 
   indicator[x]* (Cos[\[Pi] t] + Sin[\[Pi] t]) Sin[\[Pi] x];
(*données initiales propres k=1*)
ic = {u[x, 0] == Sin[\[Pi] x], 
   Derivative[0, 1][u][x, 0] ==   \[Pi] Sin[\[Pi] x]};
(* Condition aux bord de Dirichlet*)
bcc = {u[0, t] == 0, u[1, t] == 0};
(*résolution analytique de l'équation *)
sol = DSolve[{E1, ic, bcc}, u, {x, 0, 1}, {t, 0, 12}]

I got a solution, but I can't plot it !!

Grid[Partition[Table[Plot[Activate[(u /. sol[[1]])[[2]] /. Infinity -> 50][[1, 1]][[1]], {x,0, 1}, Exclusions -> None, PlotRange -> All], {t, 0, 12}], 3], 
ItemSize -> 10]

Please, how I can solve this last problem ?

Thank you so much, Mr Mariusz, for your help, please if you have time I have one more question : I want to calculate the norm $ L ^ {2} $ of the solution when $t = 12$, I defined the integral in $ L ^ {2} $ as following:

(*Symbolic*)
L2[f_] := Sqrt[Integrate[Abs[f]^2, {x, 0, 1}]];
(*numérique*)
NL2[f_] := Sqrt[NIntegrate[Abs[f[x]]^2, {x, 0, 1}]];

then I defined a function h as follows:

h[x_] := SOL[x, 12]

but when I apply the norm to $ h $ it gives error

NL2[h] (*error*)

Can you help, please !!

I'm very very sorry Mr Mariusz, but I made a big mistake at the start, I forgot to replace $ t $ with $ t_ {1} $, in the second question,

t1 = 2;
indicator[x_] := Piecewise[{{1, 0 < x < 1/2}}, 0];
(* problème du controle*)
E1 = D[u[x, t], {x, 2}] - D[u[x, t], {t, 2}] == 
indicator[x]* (Cos[2 \[Pi] t1] + Sin[2 \[Pi] t1]) Sin[2 \[Pi] x];
(*données initiales propres *)
ic = {u[x, 0] == Sin[2 \[Pi] x], 
Derivative[0, 1][u][x, 0] ==   2  \[Pi] Sin[2 \[Pi] x]};
(* Condition aux bord de Dirichlet*)
bcc = {u[0, t] == 0, u[1, t] == 0};
(*résolution analytique de l'équation *)
sol = DSolve[{E1, ic, bcc}, u, {x, 0, 1}, {t, 0, 12}]

I found the solution as usual. But, other times I can't plot my solution

Grid[Partition[
  Table[Plot[
    Activate[(u /. sol[[1]])[[2]] /. Infinity -> 50][[1, 1]][[1]], {x,
      0, 1}, Exclusions -> None, PlotRange -> All], {t, 0, 12}], 3], 
 ItemSize -> 10]

I tried a lot to find the problem but i can't, if you can help me other times, please

thank you for all your answers, I think the good question I need to ask is: how to extract the solution from DSolveValue? I did not understand the principle that you follow! other times I modified 2 to 3, and I have almost the same form of the solution, and I cannot extract the solution,

t1 = 2;
(*Définit la fonction indicatrice*)
indicator[x_] := Piecewise[{{1, 0 < x < 1/2}}, 0];
(*problème du controle*)
E1 = D[u[x, t], {x, 2}] - D[u[x, t], {t, 2}] == 
   indicator[x]*(Cos[3 \[Pi] t1] + Sin[3 \[Pi] t1]) Sin[3 \[Pi] x];
(*données initiales propres k=1*)
ic = {u[x, 0] == Sin[3 \[Pi] x], 
   Derivative[0, 1][u][x, 0] == 3 \[Pi] Sin[ 3 \[Pi] x]};
(*Condition aux bord de Dirichlet*)
bcc = {u[0, t] == 0, u[1, t] == 0};
(*résolution analytique de l'équation*)
sol2 = DSolve[{E1, ic, bcc}, u, {x, 0, 1}, {t, 0, 12}]

Activate[sol2 /. Infinity -> 1][[1, 1, 1]](*extractin de la solution de DSolveValue*)

It's not working !!

Hello Mariusz, i came back to you again please I tried to solve this system by NDSolve, but I find errors and I do not know why! if you can help me?

t1 = 2;

(Définit la fonction indicatrice)

indicator[x_] := Piecewise[{{1, 0 < x < 1/2}}, 0];



E1 = D[g[x, t], {x, 2}] - D[g[x, t], {t, 2}] ==  indicator[x]*((Cos[\[Pi] t1] - Sin[\[Pi] t1]) Sin[\[Pi] x])/(
4 \[Pi])*DiracDelta[t - t1];
ic = {g[x, 0] == Sum[Sin[k *\[Pi]*x], {k, 1}], Derivative[0, 1][g][x, 0] ==  Sum[k *\[Pi] *Sin[k *\[Pi]* x], {k, 1}]};
(*Condition aux bord de Dirichlet*)
bcc = {g[0, t] == 0, g[1, t] == 0};
(*résolution numérique de l'équation*)
sol11 = NDSolve[{E1, ic, bcc}, g, {x, 0, 1}, {t, 0, 12},  MaxStepSize -> 0.1];

Best regards,

I only changed to:

indicator[x_] := Piecewise[{{1, 0 < x < 1/2}}, 0];(* 1/2  not  0.5 !!! *)

On Mathematica 12.1 I get:

{
 {u -> Function[{x, t}, Piecewise[{{Inactive[Sum][Sqrt[2]*Sin[Pi*x*K[1]]*(Cos[Pi*t*Sqrt[K[1]^2]]*Piecewise[{{1/Sqrt[2], Inequality[0, Less, K[1], Less, 3]}}, 0] - 
          ((-1 + Cos[Pi*t*K[1]])*Piecewise[{{-(8 + 3*Pi)/(6*Sqrt[2]*Pi), Inequality[0, Less, K[1], Less, 3]}}, (Sqrt[2]*((Cos[(Pi*K[1])/2]*K[1])/(-1 + K[1]^2) + 
                (2*Sin[(Pi*K[1])/2])/(-4 + K[1]^2)))/Pi])/(Pi^2*K[1]^2) + Piecewise[{{1/(Sqrt[2]*K[1]), K[1] == 1}, {Sqrt[2]/K[1], K[1] == 2}}, 0]*Sin[Pi*t*Sqrt[K[1]^2]]), 
        {K[1], 1, Infinity}], Element[K[1], Integers] && K[1] >= 1}}, Indeterminate]]}}

It works because DSolve is exact symbolic solver, work very good only with exact numbers.