I would like to plot the solution of the Fredholm Equation $$f\left(x\right)+\frac{1}{\pi} \int_{-1}^{1} \frac{1}{1+\left(x-t\right)^2}f\left(t\right) dt=1, \ \ (|x|\leq 1)$$ I tried to use Mathematica to find a numerical solution:

PHI = DSolveValue[\[Phi][x] == 
   3 + \[Lambda] Integrate[Cos[x - s] \[Phi][s], {s, 0, Pi}], \[Phi], x]
Plot[PHI, {x, -1, 1}]

But I obtained some errors and I can't plot the numerical solution. How can I fix this problem?

I would like to plot the solution of the Fredholm Equation $$f\left(x\right)+\frac{1}{\pi} \int_{-1}^{1} \frac{1}{1+\left(x-t\right)^2}f\left(t\right) dt=1, \ \ (|x|\leq 1)$$ I tried to use Mathematica to find a numerical solution:

PHI = DSolveValue[\[Phi][x] == 1 - 1/Pi*Integrate[\[Phi][t]/(1 + (x - t)^2), 
      {t, -1, 1}], \[Phi], x]
Plot[PHI, {x, -1, 1}]

But I obtained some errors and I can't plot the numerical solution. How can I fix this problem?