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differential equations - DSolve with Absolute Value


When playing around with DSolve, I enter the following:


  DSolve[{Derivative[1][y][x] == x + Abs[x]*y[x]}, y[x], x]

The output is:


y(x)→e12(x|x|−1)∫x1K[1]e12(1−K[1]|K[1]|)dK[1]+c1e12(x|x|−1)


I would have expected a conditional which accounts for x≤0 and x>0 or something along those lines.


How is one supposed to interpret the output that is being produced? Is it a bug or am I not reading it correctly?



Answer



Testing the solution numerically shows the solution appears to be valid:



{sol} = DSolve[{Derivative[1][y][x] == x + Abs[x]*y[x]}, y, x]

SeedRandom[0];
RandomReal[{-2, 2}, 10]
Table[
 Block[{C},
  C[1] = 1;
  Derivative[1][y][x] == x + Abs[x]*y[x] /.
    (sol /. Integrate -> NIntegrate) /. Abs' -> Sign],
 {x, %}]

(*
  {0.609871, 0.532281, 0.731252, 0.265407, 1.74081, 1.90475,
   -1.04619, 0.550249, -1.59561, 0.582099}

  {True, True, True, True, True, True, True, True, True, True}
*)


In response to Amzoti's comment, I'm not so sure about there being a nice solution, if x is assumed to be complex instead of real, which is how it is treated in Mathematica. If we specify that x is real, we do get a nice solution (and much more quickly):


Assuming[{x ∈ Reals},

  {sol} = DSolve[{Derivative[1][y][x] == x + Abs[x]*y[x]}, y, x]
  ];
y[x] /. sol // PiecewiseExpand

Mathematica graphics


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