Laplace and ODE - what is wrong?

[WalterSchrabmai]

Dear friends,

I have here a simple example, but in MC15 I do not get the same results as in the example.

WHo can help?

Thanks

Walter

[WalterSchrabmai]

I have found my misunderstanding of Laplace transform. Only f(t) must be tranformed, see MC15 sheet.

[ValeryOchkov]

Do you know this:

[WalterSchrabmai]

Dear Valery,

I want to solve a ODE by LaplaceTransformation. I have f(t) and in MMA the results is as follows. (see pic)

ANd in MC 15 I do get a other form. Is that the same? Thanks for help

Walter

[WalterSchrabmai]

Here I could solve the ODE with laplace in MMA successfully. BUT in MC it does n ot work .

WHY? Any advices are welcome.

Thanks

Walter

[Fred_Kohlhepp]

Mathcad is primarily a numeric calculator; the symbolic processor, while convenient, is not always as good as we'd like.

Accepting some help from the books and literature allows us to do the laplace transform of the Heaviside function by ourselves:

Can you solve it then?

[WalterSchrabmai]

Dear Fred,

thanks a lot for your fast response.

When I transform the f(t) with MMA I got this result. Are you sure that you did not miss the Theta(-tau)?

Thanks for your reply.

[ValeryOchkov]

There is (was) a lot of errors at Laplace Trans in Mathcad 14/15 (with MuPad not Maple Symbolic Engine).

[WalterSchrabmai]

Yes I think that is still a bug. See inv transform of f(t) in pic.

[ValeryOchkov]

Walter Schrabmair wrote:

Yes I think that is still a bug. See inv transform of f(t) in pic.

This is one reason why our (with V.Korobov) book has a titel "Chemical Kinetics with Mathcad and... Maple"

[WalterSchrabmai]

thanks Valery, I tried it with Maple v15 but have problems in translate the MMA 8.0 sheet into Maple.

I have asked a help question in http://www.mapleprimes.com/questions/130265-Who-Can-Help-Me-In-Transform-A-Simple?sq=130265

Maybe there is a helping hand for that problem on the Maple forum.

thansk

walter

[Fred_Kohlhepp]

I'm flailing a bit, and my version 14.0 is having serious problems with the symbolic solver.

See if this makes sense:

[WalterSchrabmai]

Looks good but got still an error that the X1 or X2 fn is not defined - even that in MC 15 the red invlap lines signals that is just to big to display but should be able to store in fn.

Please take a look.

Thanks a lot

[WalterSchrabmai]

THis Problem with Laplace substitution can also not be solve in MathCAD 15 M 020. WHat a shame!

[Werner_E]

The real problem seems not to be that the expression is too big but rather that Mathcad simply is not capable of doing that specific inverse Laplace transform. After definition of all variables the expression is not too big anymore but nevertheless useless.

I agree - it's a shame

[WalterSchrabmai]

Thanks Werner, yes I agree to you. Let`s wait until PRIME 6.0, maybe they will correct this bug.

[Werner_E]

Prime 6 already!? Thats what I call optimism and positive thinking 😉

But then, I won't bet on it.