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FullSimplify Question


Hello.  Hate to ask, but does anyone know what the issue is in the
following?

This equation basically is 5 when t is less than 0, and 1 when t>=0

equ = 5 - 4*UnitStep[t]

If I ask to "Simplify" this, by using the Sign function, I get the
following, which is not even close

FullSimplify[equ, TransformationFunctions -> {Sign}]

1 - UnitStep[t]

The above is 1 when t<0, and 0 when t>=0.

I have to use both functions "FourierTransform" and
"InverseFourierTransform" to get the correct form.

3 - 2*Sign[t]

My question is that I can't figure out why FullSimplify was so far off.
Would Mathematica have been better to leave the expression unevaluated so as to use
other methods?  When the equation is more complicated, it may be harder to
spot such "errors."

Thanks for any insight.

Dana DeLouis
delouis at bellsouth.net


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