In order to solve the following Euler Differential Equation I write in the notebook:

Assuming[{y∈\[DoubleStruckCapitalR], n>0,x>0},DSolve[x^2 y''[x]+x  y'[x]- n y[x]==0,y[x],x]]

And I get as an output:

{{y[x]->Subscript[[ConstantC], 1] Cosh[Sqrt[n] Log[x]]+I Subscript[[ConstantC], 2] Sinh[Sqrt[n] Log[x]]}}

On the other hand when Mathematica solves the above equation with a specific value for n, for example $n=4$ , it returns the desired solution...

How can I fix this problem, please?

  • 1
    $\begingroup$ First, you may see that \[DoubleStruckCapitalR] is colored with blue, which means that this has no built-in meaning and doesn't represent the set of real numbers. You should use Reals or type with Esc``reals``Esc. $\endgroup$ – SneezeFor16Min Jul 2 '20 at 7:21
  • $\begingroup$ @SneezeFor16Min,Thanks but if I write Reals, it does not work either.... $\endgroup$ – dmtri Jul 2 '20 at 7:27
  • $\begingroup$ You could specify initial conditions y[1] == y1 and y'[1] == yP1. $\endgroup$ – Natas Jul 2 '20 at 7:29

First of all, DSolve does not really take assumptions. (What I mean, DSolve seems to ignore assumptions)

Euler ODE can have 3 different solutions depending on roots $r_1,r_2$ of the characteristic equation. For one repeated root, solution is $y=c_1 x^r+ c_2 x^r \ln x$, and for 2 real and distinct roots, solution is $y=c_1 x^{r_1}+ c_2 x^{r_2}$ and for two complex conjugate roots, solution is $y=c_1 x^{\alpha+i\beta} + c_2 x^{\alpha-i\beta}$

For your case, the roots are $r_1 = -\sqrt{n},r_2 = \sqrt{n}$. Hence for $n>0$ then we are in the second case (2 real distinct roots).

The characteristic equation for Euler ode is found by substituting $y=A x^r$ into the ode and then solving for $r$

You can obtain, for n>0 the solution you want this way

ode = x^2 y''[x] + x y'[x] - n y[x] == 0
sol = DSolve[ode, y[x], x]

enter image description here

sol = TrigToExp[sol]

enter image description here

sol = Collect[sol, { x^-Sqrt[n], x^Sqrt[n]}]

enter image description here

We are free to rename constants as we want. Hence

sol = sol /. {C[1]/2 - (I C[2])/2 -> C[3],    C[1]/2 + (I C[2])/2 -> C[4]}

enter image description here


 ode /. y -> Function[{x}, x^-Sqrt[n] C[3] + x^Sqrt[n] C[4]] // Simplify

 (* True *)


what is the characteristic equation?

ode = x^2 y''[x] + x y'[x] - n y[x] == 0;
charEquation = ode /. y -> Function[{x}, A*x^r]

enter image description here

Since $A$ and $x^r$ are non zero, we can divide by them and the above simplifies to

Solve[-  n  +  r + (-1 + r) r == 0, r] 

which gives the solutions

enter image description here

And since $n>0$ then we have two real distinct roots.

  • $\begingroup$ Could you clarify what you mean by saying that DSolve does not really takes assumptions? DSolve has an option Assumptions. $\endgroup$ – Natas Jul 2 '20 at 7:30
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    $\begingroup$ @Natas I mean, from experience, I found that assumptions most of the time are ignored by DSolve. Just because you pass assumptions to it, does not mean it will use them. $\endgroup$ – Nasser Jul 2 '20 at 7:33
  • $\begingroup$ OK, I agree. Assumptions cause many problems and are poorly implemented. $\endgroup$ – Natas Jul 2 '20 at 7:38
  • 1
    $\begingroup$ Here we have no constant coefficients, so what is the characteristic equation? $\endgroup$ – dmtri Jul 2 '20 at 10:14
  • $\begingroup$ @dmtri i've updated the answer with the char equation. $\endgroup$ – Nasser Jul 2 '20 at 10:50

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