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I'm trying to use Mathematica to evaluate many values of $f(x,y) $, like $f(-\pi,\pi) $ for

$$f(x,y) = \left( \frac{-1}{4}\sin \left( \frac{x-y}{2} \right)\sin y \right)\left( \frac{-5}{4}\sin \left( \frac{x-y}{2} \right)\sin y-\cos \left( \frac{x-y}{2} \right)\cos y \right) \\-\frac{1}{4}\left( \sin \left( \frac{x-y}{2} \right)\sin y+\cos \left( \frac{x-y}{2} \right)\cos y \right).$$

So I input :

f[x_, y_] := ((-1/4) sin[(x - y/2)] sin[y]) 
             ((5/4) sin[(x - y/2)] sin[y] - (-1/4) cos[(x - y/2)] cos[y]) 
             - (1/4) (sin[(x - y/2)] sin[y] + cos[(x - y/2)] cos[y])


f[-\[Pi], \[Pi]]

Mathematica then outputs :

1/4 (-cos[-((3 \[Pi])/2)] cos[\[Pi]] - 
    sin[-((3 \[Pi])/2)] sin[\[Pi]]) - 
 1/4 sin[-((3 \[Pi])/
    2)] sin[\[Pi]] (1/4 cos[-((3 \[Pi])/2)] cos[\[Pi]] + 
    5/4 sin[-((3 \[Pi])/2)] sin[\[Pi]])

But I want my final answer to be a number. Why didn't Mathematica evaluate the trigonometric functions? Thank you.

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I changed some "=" to "-" in the first formula but otherwise left it alone, even though it disagrees with your Mathematica input in quite a few places. You might want to check that you typed the correct formula into MMA. In particular, the two of you may disagree sharply about the interpretation of "x-y/2", because it rarely equals $\frac{x-y}{2}$. –  whuber Jan 9 '13 at 0:13
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closed as too localized by Mr.Wizard Jan 9 '13 at 7:11

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1 Answer

up vote 3 down vote accepted

It works for me on V 9

f[x_, y_] := ((-1/4) Sin[(x - y/2)] Sin[y]) ((5/4) Sin[(x - y/2)] Sin[y] - 
             (-1/4) Cos[(x - y/2)] Cos[y]) - (1/4) (Sin[(x - y/2)] Sin[y] + 
             Cos[(x - y/2)] Cos[y])

In[45]:= f[-Pi, Pi]
Out[45]= 0

(ps. it helped to write Sin instead of sin and Cos instead of cos :)

Mathematica's commands, symbols and functions start with UpperCase. You are on the right track of using lowerCase, but you should do that for your own variables and symbols, not for what is meant to be Mathematica's own.


 Length@Cases[UpperCaseQ[#] & /@ StringTake[Names["System`*"], 1], True]

But almost all those which are not UpperCase are actually these strange symbols and they do not really count:

Mathematica graphics

So you can see, that Mathematica uses UpperCase first letter for its own use, which is a good thing.

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Thanks a lot! I can't believe I spent an hour because I didn't realize Upper Case Was Necessary. –  dresserse Jan 9 '13 at 0:04
You could have asked WolframAlpha: f[x_, y_] = ReleaseHold@ WolframAlpha[ "((-1/4) sin[(x-y/2)] sin[y]) ((5/4) sin[(x-y/2)] sin[y]-(-1/4) \ cos[(x-y/2)] cos[y])-(1/4) (sin[(x-y/2)] sin[y]+cos[(x-y/2)] \ cos[y])", {{"Input", 1}, "Input"}] Also just WolframAlpha["((-1/4) sin[(x-y/2)] sin[y]) ((5/4) sin[(x-y/2)] \ sin[y]-(-1/4) cos[(x-y/2)] cos[y])-(1/4) (sin[(x-y/2)] \ sin[y]+cos[(x-y/2)] cos[y])"] gives a nice overview of your function –  Rolf Mertig Jan 9 '13 at 0:05
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