I am creating a problem with a symbolic answer. I thought the easy way to do this would be just to create my own function X(w) = w, and use it in the answer. The Fourier transform of x(n) + x(n-1) is X(w) + exp(-j*w)*X(w). It doesn't really matter what X(w) is as long as I can evaluate it. See the code below.

DOCUMENT();

# modified to use j for i = sqrt(-1)

$complexJ =1; # 0 => i, 1 => j

$I = ($complexJ)? 'j': 'i';

loadMacros(

"PGstandard.pl", # Standard macros for PG language

"MathObjects.pl",

($complexJ) ? "contextComplexJ.pl" : "",

#"source.pl", # allows code to be displayed on certain sites.

"PGcourse.pl", # Customization file for the course

"parserFunction.pl",

);

# DTFT of aperiodic series geometric series

TEXT(beginproblem());

# Show which answers are correct and which ones are incorrect

$showPartialCorrectAnswers = 1;

##############################################################

#

# Setup

#

#

Context("Complex");

Context()->variables->are(w=>'Real');

#Context()->variables->are(n=> 'Real',w=>'Real'); #Context()->variables->remove(z); # only variables should be n and w

parserFunction("X(w)" => "w");

$n0 = random(1,9,1); # delay of sequence

$c1 = random(1,9,1); # gain of x(n)

$c2 = random(1,9,1); # gain of x(n-$n0)

#$Xp = Formula("$c1*X(w) - $c2*e**((-1)*j*w$n0)*X(w)")->reduce->with(test_at => [[0,0],[0,1],[0,2],[0,3]]); # used with two variables n and w

$Xp = Formula("$c1*X(w) - $c2*e**((-1)*j*w$n0)*X(w)")->with(test_at => [[0],[1],[2],[3]]);

##############################################################

#

# Text

#

#

Context()->texStrings;

BEGIN_TEXT

This problem is related to 4.17a in the text.

$PAR

Let \( x(n) \) be an arbitrary signal, not necessarily real valued, with Discrete Fourier transform, \(X(\omega ) \),

$BR Express the Discrete Fourier transform of the following signal in terms of \(X(w) \),

$BR Use \(w \) for \(\omega\)(omega) to make typing easy.

\[

y(n) = $c1*x(n) - $c2*x(n-$n0),

\]

$PAR

$BR \(Y(\omega )= \) \{ $Xp ->ans_rule(40)\}

END_TEXT

Context()->normalStrings;

##############################################################

#

# Answers

#

#

ANS($Xp->cmp());

ENDDOCUMENT();

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