Re: NDSolve and error
- To: mathgroup at smc.vnet.net
- Subject: [mg92563] Re: NDSolve and error
- From: Peter Pein <petsie at dordos.net>
- Date: Sun, 5 Oct 2008 06:06:33 -0400 (EDT)
- References: <gc7fql$elo$1@smc.vnet.net>
dodatki at poczta.fm schrieb: > Hello! > Lately I wrote simple code in Mathematica > > sol[p_] := > NDSolve[{Sqrt[ > 1 + (y'[t])^2] == (x^(p - 1) + (y[x])^(p - 1)*y'[x])/(x^p + > y[x]^p)^(1 - 1/p), y[1] == 6}, y, {x, 1, 4}] > > but when I typed sol[2] the following kind of error was shown: > > NDSolve::"ndnum": "Encountered non-numerical value for a derivative at > \ > \!\(x\) == \!\(4.587812868332132`*^-296\)." > > May I know what I'm doing wrong? Or how can I improve that code to > work correctly? > Thank you in advance > John > Hi John, try to replace y'[t] in the call to Sqrt by y'[x], and you'll get an exact solution for p=2: In[3]:= DSolve[{Sqrt[Derivative[1][y][x]^2 + 1] == (x^(p - 1) + y[x]^(p - 1)*Derivative[1][y][ x])/(x^p + y[x]^p)^(1 - 1/p), y[1] == 6} /. p -> 2, y, x] Out[3]= {{y -> Function[{x}, 6*x]}} Peter