       Re: Diff. Equations with "Changeable" Parameters

• To: mathgroup at smc.vnet.net
• Subject: [mg51680] Re: Diff. Equations with "Changeable" Parameters
• From: "Jens-Peer Kuska" <kuska at informatik.uni-leipzig.de>
• Date: Fri, 29 Oct 2004 03:38:58 -0400 (EDT)
• Organization: Uni Leipzig
• References: <clpsou\$abt\$1@smc.vnet.net>
• Sender: owner-wri-mathgroup at wolfram.com

```Hi,

a) you do not solve
> NDSolve[{x'[t]=y[t],y=0}, {y'[t]=x+Sin[t]+c*y[t]},{x,y},{t,0,2Pi}]

because it is full of errors, for equations you have to use Equal[] == and
not Set[] =,
there is no initial condition for x and in the last differential equation
a x[t] is missed.

b) in your question c is simply a function of y and you should define a
function c[y] with
c[y_?NumericQ] /; y < -1 := 0.0
c[y_?NumericQ] /; -1 <= y <= 1 := 1.
c[y_?NumericQ] := 2.0

and type the correct version of the NDSolve[] command

NDSolve[{x'[t] == y[t], y == 0, x == 0,
y'[t] == x[t] + Sin[t] + c[y[t]]*y[t]}, {x, y}, {t, 0, 2Pi}]

Regards

Jens

"Krunom Ilicevic" <krunom at hotmail.com> schrieb im Newsbeitrag
news:clpsou\$abt\$1 at smc.vnet.net...
>I have solved diff. equations in this kind of a way:
>
>
> and parameter c was 1, but how to write this algorithm if c is:
>
> c=0, if y<=-1
>
> c=1, if -1<y<1
>
> c=2, if y>=1
>
> How to include this variable parameter c in my NDSolve method?
>
>
>
> Thanks.
>
>

```

• Prev by Date: Re: full QR decomposition
• Next by Date: Re: full QR decomposition
• Previous by thread: Re: Re: Diff. Equations with "Changeable" Parameters
• Next by thread: Does anybody out there know how to use the categorylists function for p-variate date