Re: NDSolve Question

• To: mathgroup at smc.vnet.net
• Subject: [mg86488] Re: NDSolve Question
• From: Jerry <Jer75811 at yahoo.com>
• Date: Wed, 12 Mar 2008 00:14:54 -0500 (EST)
• References: <fr5nrk\$1ia\$1@smc.vnet.net>

```Sir, the + signs are still missing so something must be
wrong on my end. Thanks for your patience.

Bob Hanlon wrote:
> The e-mail that was quoted in your reply was not what I sent. Some characters (plus signs) got lost on the way. The function was modified to x[t] plus 2 in the DSolve example and to q plus 2 in definition of f. I have resent the input/output.
>
> 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 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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