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I have obtained the following form as a part of a computation:

k (Quantity[0, "Kilohertz"]) + Quantity[1/500, "Kilohertz"]

This is typeset as

Here, k is an atom (representing an integer in my computation). Obviously I would like this to simplify to Quantity[2, "Hertz"], but I cannot figure out how to cause the term with k to vanish. Neither FullSimplify nor UnitSimplify have any effect here:

My best guess is that Mathematica doesn't want to assume things about k, but given that the magnitude of the unit is 0, it should still simplify away.

How can I simplify this as I would expect?

thirtythreeforty
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2 Answers2

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You can do this:

Unprotect[Quantity]
Quantity /: Times[Quantity[mag_, unit_], seq__] := 
Quantity[Times[mag, seq], unit]

This will leads to your desired result, and also works for symbolic Times in Quantity

vapor
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    That's a good idea. Is it wise to edit builtin rules like Quantity? Also, would Quantity[0, __] := 0 be a better rule or worse? (I don't know general conventions, thanks.) – thirtythreeforty Apr 05 '16 at 05:23
  • @thirtythreeforty your idea also "edits" the built-in function (requires Unprotect[ Quantity]) . Which to do depends on your need. – vapor Apr 05 '16 at 05:40
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Had the same problem and solved it by simply using a replacement rule.

k (Quantity[0, "Kilohertz"]) + Quantity[1/500, "Kilohertz"]

UnitConvert[Out[1] /. Quantity[0, "Kilohertz"] -> 0, "Hertz"]

Here's what it looks like in a notebook:

enter image description here

T. Zilla
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    People here generally like users to post code as Mathematica code instead of images or TeX, so they can copy-paste it. It makes it convenient. –  Jun 05 '16 at 08:35
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  • Louis, thanks for the formatting suggestion. – T. Zilla Jun 05 '16 at 17:37