Re: NDSolve Question

• To: mathgroup at smc.vnet.net
• Subject: [mg86427] Re: NDSolve Question
• From: Jerry <Jer75811 at yahoo.com>
• Date: Tue, 11 Mar 2008 02:57:53 -0500 (EST)
• References: <fr2me7\$o2m\$1@smc.vnet.net>

```Sir, I tried your suggestions but I never get a plot unless
I use the init. condition x[0] == 1 as suggested by
jwmerrill. So I guess I still don't understand the issues.
Thanks.

Bob Hanlon wrote:
> f[q_?NumericQ] := q;
>
> This function is not very useful as an example since
>
> DSolve[{x'[t] == x[t], x[0] == 0}, x[t], t][[1]]
>
> {x[t] -> 0}
>
> With a slight change
>
> DSolve[{x'[t] == x[t]  2, x[0] == 0}, x[t], t][[1]]
>
> {x[t] -> 2*(-1  E^t)}
>
> f[q_?NumericQ] := q  2;
>
> Note that NumericQ is better than NumberQ so that f will evaluate with arguments like Pi or E
>
> f[3 Pi]
>
> 2  3*Pi
>
> sol = NDSolve[{x'[t] == f[x[t]], x[0] == 0}, x, {t, 0, 1}][[1]];
>
> Plot[x[t] /. sol, {t, 0, 1}]
>
>
> Bob Hanlon
>
> ---- Jerry <Jer75811 at yahoo.com> wrote:
>> Sir, I tried this and I only get plot axes, no graph.
>>
>> f[q_?NumberQ] := q
>> sol = NDSolve[{x'[t] == f[x[t]], x[0] == 0}, {x}, {t, 0, 1}]
>> Plot[x[t] /. sol, {t, 0, 1}]
>>
>> In place of {x} in sol I tried x and x[t] with no change.
>> In Plot I tried x instead of x[t], no help.
>>
>> Can you give me a successful example? Thanks.
>>
>>
>>
>>
>> David Park wrote:
>>> I found the answer, which is to use:
>>>
>>> f[q_?NumberQ]:= ...
>>>
>>> which prevents an initial evaluation.
>>>
>
>

```

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