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# Mathcad need denom and numer funcitons

## Mathcad need denom and numer funcitons

(OP)
I am often doing control theory work where I have a transfer function in the s or z domain.  There is no easy way to automatically get the numerator or denominator of the transfer function.  This must be done manually which is a pain and is sometimes forgotten which create errors.  There should be numer and denom functions or is there some other method of extracting the numerator or denominator? I have Mathcad 13.

### RE: Mathcad need denom and numer funcitons

Numer and denom are two of the new functions added in MC14.

Peter

### RE: Mathcad need denom and numer funcitons

(OP)
That is good to know.  Its about time these functions are part of Mathcad.

Peter Nachtwey

### RE: Mathcad need denom and numer funcitons

denom and numer have been part of Mathcad since at least v11 and features in one of the Quicksheets.

Try dn(x,y):=denom(y/x^2)-> and nm(x,y):=numer(y/x^2)->

(they are symbolic operators)

Stuart

### RE: Mathcad need denom and numer funcitons

Interesting, those do indeed work in MC11, 13 and 14.  According to the MC14 release notes, Numer and Denom are "new features" in MC14 (See #2 under symbolics enhancements).

Peter

### RE: Mathcad need denom and numer funcitons

(OP)
Wow!  Mathcad sure has kept that a secret!  I can't find numer or denom in the help at all but I tried it and it works!  I can now generator coefficients for by difference equations automatically from the z domain transfer function. I use numer and denom and then coeffs,z.

I will try out the numer and denom functions with polyroots tomorrow to see if I can easily generate poles and zeros.  Then my pole/zero plots will update automatically too.

I wonder what other secrets there are.

My next complaint is that often Mathcad produces transfer functions where the sign is wrong for both the numerator and the denominator.   Example -x/-y.   Is there and easy way to make this x/y?

After that I often see functions the look like this  (-b+a)/3 instead of (a-b)/3.  I know this is small stuff but I would like to use my Mathcad examples in a book someday and it would be nice if I didn't have to manually edit so much.

Peter Nachtwey

### RE: Mathcad need denom and numer funcitons

versions prior to M14 used Maple and contained many undocumented Maple features.  Don't know if there's any compendium of those anywhere.

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