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## Re: Problem with 'substitute' in Mathcad15

I will try to do as you advise.

## Re: Problem with 'substitute' in Mathcad15

I can now get 'substitute' to appear by using the symbolic keyword menu, but now 'i' is an undefined variable. See teh attached file.

## Re: Problem with 'substitute' in Mathcad15

The 'vector' arrow was added by Mathcad, not me. I quite understand that omega is not a vector.

## Re: Problem with 'substitute' in Mathcad15

Yes, I discovered that one has to use the menu.  I wish the Help was a bit more forthcoming on this subject, instead of showing very simple examples which few would need help with.

## Re: Problem with 'substitute' in Mathcad15

Not sure what you are looking for at the end of the day, but wouldn't Terry's approach be all you need? Without any symbolic "substitute".

Or is your goal to apply some further symbolic manipulations on F after the substitution?

P.S.: I would suggest to use a literal index for your constants omega.1, etc. instead of a vector index. The way you defined those constants you had defined a 5 element vector omega (first element being 0 as Mathcads first index is by default 0, not 1). I guess you did not intend to define a vector omega, right?

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## Re: Problem with 'substitute' in Mathcad15

Thank you. Do you mean that the subscript for omega-1 should be '.1', and for omega-n should be '.n'? Is this syntax explained anywhere?

How did you decide about the need for 'Independent variable range' and how to define it?

I think this is getting impracticably complicated, but I see it produces what looks like the correct magnitude response.

## Re: Problem with 'substitute' in Mathcad15

That looks a lot simpler. BIG hidden error using the wrong keys to get the subscripts. That's what comes of using Mathcad only infrequently. When I've used it intensively in the past, I was defining vectors very often.

## Re: Problem with 'substitute' in Mathcad15

That's correct, i is generally an undefined variable in Mathcad.

In order to get i, the unit of imaginary numbers, you have to type 1i.

But if you need 2*pi*i, you can also type 2i*p[Ctl-G]. The important part is that i is immediately preceded by a number.

Success!

Luc

## Re: Problem with 'substitute' in Mathcad15

In the form I used to solve your problem, I cannot use a vector as an independent variable of a function and use constants with the same name as the independent variable. I can define constants as elements of a vector that has a name different from the independent variable. The constant pulsation, for example 1 / (RC), I define it as: omega.1: = 1 / (R C). I have never used vectors as independent variables.

In recent years I have published here, in this community, some files related to analog and digital filters, for example this:https://community.ptc.com/t5/PTC-Mathcad/4-HIGH-PASS-I-order-DIGITAL-FILTERS-EQUIVALENT-TO-LINEAR-CL...
but there are others. In these files the criteria for choosing sampling frequencies are missing. Gap completed in new versions.