symbolic calculation

2011 3 31
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Why do I get
>> a = sqrt(sym(2))
a =
1/2
2
instead of
a = 2^(1/2)
And I got >> H = sym(hilb(2))
H =
[ 1. 0.500000000000000]
[ ]
[0.500000000000000 0.333333333333333]
instead of the fraction version. Is there any expert who can tell me how to set this? I use the mathlab 2010b.

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Walter Roberson
Walter Roberson 2011-3-31

1 个投票

For the first point, try turning off pretty printing
evalin(symengine, 'PRETTYPRINT := FALSE');
I do not promise that it will work, though. If it doesn't, then
char(a)
should show you the linear form.
I don't know why the decimal form is given for the hilbert matrix. You could try, though,
evalin(symengine, 'linalg::hilbert(2)');
See the documentation

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jack
jack 2011-3-31
Hmm, I don't know why I got the following when I tried your suggestion.
>> evalin(symengine, 'PRETTYPRINT := FALSE')
??? Undefined function or variable 'symengine'.
and
evalin(symengine, 'linalg::hilbert(2)')
??? Undefined function or variable 'symengine'.
jack
jack 2011-3-31
I was wondering that if there is a way for converting between the decimal and the fraction forms for the "symbol" case I mentioned in the question.
Walter Roberson
Walter Roberson 2011-3-31
Which Matlab version are you using? If you are using a version old enough that it uses Maple instead of MuPad, then as best I recall sym() did not automatically convert to rational back then (but I could be wrong.)
If you have the Maple engine, try
maple( subs('convert(H,rational)','H', hilb(2)) )
jack
jack 2011-3-31
The version I used is MATLAB 7.11.0.584(R2010b). I still got
>> maple( subs('convert(H,rational)','H', hilb(2)) )
ans =
[ 1. 0.500000000000000000]
[ ]
[0.500000000000000000 0.333333333333333315]
I find that I don't have the symbolic math toolbox in my matlab. Maybe this is the reason.
Walter Roberson
Walter Roberson 2011-3-31
2010b cannot use Maple. If it doesn't know symengine, the implication would be that you do not have the symbolic toolbox, except then I would have expected it to fail on sym . I'm surprised you are getting anywhere with those commands if you don't have the symbolic toolbox.
jack
jack 2011-3-31
Hmm, that's the key point. Everything will be OK with the symbolic toolbox.

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jack
jack 2011-3-31

0 个投票

The key point is that the MATLAB one uses needs to have the symbolic toolbox. Otherwise, one can not go further for the symbolic calculation.

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