Plot directional field with a initial vlue

Question

I do not have enough reputation to comment on other questions, hence I have to ask my own one

I want to solve the following nonlinear first order ODE,
$$(1 + x) (0.5 y[x]^{-0.5} (x - y[x])^{0.5} - 0.5 y[x]^{0.5} (x - y[x])^{-0.5}) == -y[x] (x - y[x])/y'[x]$$ As it does not have a closed form solution, I want to plot the directional field and pick a particular line corresponding to an initial value.

Here is my code,

f[x_] = {
  {NDSolveValue[{(1 + x) (0.5 y[x]^-0.5 (x - y[x])^0.5 - 
         0.5 y[x]^0.5 (x - y[x])^-0.5) == -y[x] (x - y[x])/y'[x], 
     y[1] == 0.5}, y[x], {x, 0, 2}]},
  {\[Placeholder]}
 }

But I get errors:

Power::infy: Infinite expression 1/0. encountered. >>
NDSolveValue::ndnum: Encountered non-numerical value for a derivative at x == 1.`. >>

I think I have seen others with similar functional form (potentially 1/0) plot their directional fields, is there anyway to get around here?

If I can do this first step(solving the ODE numerically), then I can plot it,

g1=Plot[f[x], {x, 0, 2},
   PlotStyle -> Directive[Thick, Red], 
   PlotRange -> {{-0.999, 2}, Automatic}, 
   PlotPoints -> 100
 ] 

then do the streamplot as it is called,

  g2=StreamPlot[{1, (-y[x] (x - y[x]))/((1 + x)[
      0.5 y[x]^-0.5 (x - y[x])^0.5 - 0.5 y[x]^0.5 (x - y[x])^-0.5])}, {x, 0, 2}, {y, 0, 1}]

and then finally showing them both

 Show[g1,g2]

The code above is what I modified from other's, his code does works. I changed the function into mine, and changed the range of x, that is all. Can someone help me figure it out?

The directional field should looks like this, but I want to highlight a red curve corresponding to a initial condition.

Also, can I add lines like y=x and y=x/2 on this plot?

Thanks in advance!

Answer 1

Only a partial solution.NDSolve can't solve with this initial conditions y(1) = 0.5

sol = NDSolve[{(1 + x)*(0.5*y[x]^(-0.5)*(x - y[x])^0.5 - 
0.5*y[x]^0.5*(x - y[x])^(-0.5)) == -y[x]*(x - y[x])/y'[x], 
y[0.99] == 0.5}, y, {x, 0, 2}, AccuracyGoal -> 30, 
PrecisionGoal -> 30, WorkingPrecision -> 55] // Quiet; 
ODEequations[x_] := y[x] /. sol;
g1 = Plot[{x, x^2, ODEequations[x]}, {x, 0, 2}, PlotRange -> {0, 2}, 
PlotLegends -> "Expressions"] // Quiet;
g2 = StreamPlot[{1, (-y*(x - y))/((1 + x)*(0.5*y^(-0.5)*(x - y)^0.5 - 
0.5*y^0.5 *(x - y)^(-0.5)))}, {x, 0, 2}, {y, 0, 1}] // Quiet;
Show[g1, g2]

The directional field looks another like yours. I can not explain this. Such a solution better than none :)