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# Re: DSolve test

89 views

### Bob Hanlon

May 12, 2014, 12:42:00 AM5/12/14
to

You did not say what bug or with which version. Version 9 gives a correct
solution for t >= 0

\$Version

"9.0 for Mac OS X x86 (64-bit) (January 24, 2013)"

sol = DSolve[
{u'[t] == 2*Sqrt[u[t]], u[0] == 0},
u[t], t][[1, 1]]

u[t] -> t^2

u'[t] == 2*Sqrt[u[t]] /.
NestList[D[#, t] &, sol, 1] //
Simplify[#, t >= 0] &

True

u[0] == 0 /. (sol /. t -> 0)

True

Bob Hanlon

On Fri, May 9, 2014 at 2:07 AM, <
carlos.felippa%colora...@gtempaccount.com> wrote:

> Hi, could somebody try this in Mathematica 9 to see if the bug is fixed?
>
> sol = DSolve[{u'[t] == 2*Sqrt[u[t]], u[0] == 0}, u[t], t]; Print[sol];
>
> Thanks.
>
>

### Murray Eisenberg

May 12, 2014, 12:43:42 AM5/12/14
to
What alleged bug did you have in mind?

Did something not work as expected with Print in an earlier version. And why use print at all? Just the DSolve expression by itself will give the same output. In Mathematica 9.0.1, this is:

DSolve[{u'[t] == 2*Sqrt[u[t]], u[0] == 0}, u[t], t]
(* {{u[t]->t^2}} *)

which, of course, is valid only for t >= 0.

On May 9, 2014, at 2:07 AM, carlos.felippa%colora...@gtempaccount.com wrote:

> Hi, could somebody try this in Mathematica 9 to see if the bug is fixed?
>
> sol = DSolve[{u'[t] == 2*Sqrt[u[t]], u[0] == 0}, u[t], t]; Print[sol];
>
> Thanks.
>

Murray Eisenberg mur...@math.umass.edu
Mathematics & Statistics Dept.
Lederle Graduate Research Tower phone 240 246-7240 (H)
University of Massachusetts
710 North Pleasant Street
Amherst, MA 01003-9305

### Bill Rowe

May 12, 2014, 12:45:03 AM5/12/14
to
On 5/9/14 at 2:07 AM, carlos.felippa%colora...@gtempaccount.com
wrote:

>Hi, could somebody try this in Mathematica 9 to see if the bug is
>fixed?

>sol = DSolve[{u'[t] == 2*Sqrt[u[t]], u[0] == 0}, u[t], t];
>Print[sol];

I don't see any evidence of a bug

In[1]:= sol = DSolve[{u'[t] == 2*Sqrt[u[t]], u[0] == 0}, u[t], t]

Out[1]= {{u[t] -> t^2}}

In[2]:= \$Version

Out[2]= 9.0 for Mac OS X x86 (64-bit) (January 24, 2013)

### Tomas Garza

May 12, 2014, 12:45:23 AM5/12/14
to

What bug? The solution is all right, ain't it? (V.9.0.1)
sol=DSolve[{u'[t]==2*Sqrt[u[t]],u[0]==0},u[t],t]; Print[sol]
{{u[t]->t^2}}
Of course, you must delete the last semicolon in the input cell if you want to see the solution printed.
-Tomas

> From: carlos.felippa%colora...@gtempaccount.com
> Subject: DSolve test
> To: math...@smc.vnet.net
> Date: Fri, 9 May 2014 02:07:27 -0400
>
> Hi, could somebody try this in Mathematica 9 to see if the bug is fixed?
>
> sol = DSolve[{u'[t] == 2*Sqrt[u[t]], u[0] == 0}, u[t], t]; Print[sol];
>
> Thanks.
>

### Sasa B

May 12, 2014, 12:46:04 AM5/12/14
to

First, all the best to one of the most influential mechanics teacher of our
time, I'm glad to give this minor help :)

Bug doesn't seem fixed

sol = DSolve[{u'[t] == 2*Sqrt[u[t]], u[0] == 0}, u[t], t]; Print[sol];

returns

{{u[t]->t^2}}

(Mathematica 9, Intel 4570)

Aleksandar Borkovic