Bug in symbolic toolbox?

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Junior 2011-8-18

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https://ww2.mathworks.cn/matlabcentral/answers/13980-bug-in-symbolic-toolbox

Hi all, I am experiencing a strange behavior of the sym function. The result of sym('I') is 'I'. While the result of sym('A+I') is 'A+i' with an unwanted lower case letter of 'I'.

Has anyone experienced such a behavior? Is it a bug? and if so is there any workaround?

Thanks, Junior

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Answer 1

Walter Roberson 2011-8-18

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https://ww2.mathworks.cn/matlabcentral/answers/13980-bug-in-symbolic-toolbox#answer_19138

在 MATLAB Online 中打开

http://www.mathworks.com/help/toolbox/mupad/getstarted/perform-computations-numbers.html

In the input regions MuPAD recognizes an uppercase I as the imaginary unit (-1)^(1/2). In the output regions, MuPAD uses a lowercase i to display the imaginary unit

http://www.mathworks.com/help/toolbox/mupad/quickref/math.html

I – imaginary unit sqrt(-1) (see DOM_COMPLEX for details)

http://www.mathworks.com/help/toolbox/mupad/datatypes/DOM_COMPLEX.html

Complex numbers can be constructed by typing in the corresponding expression, such as 3+4*I. The keyword for typing the imaginary unit I is I (a capital letter i).

Thus, what you are seeing is not a bug.

I know that in Maple it is possible to change the representation letter, but I do not at the moment see any corresponding mechanism in MuPad. (In Maple, it would be by using interface(imaginaryunit=SYMBOL) )

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Junior 2011-8-19

Thanks Walter for enlightening me on this matter. The conclusion then is that mupad and standard matlab differ in this respect. For me, the imaginary unit was "i" and not "I".

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Answer 2

Sean de Wolski 2011-8-18

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https://ww2.mathworks.cn/matlabcentral/answers/13980-bug-in-symbolic-toolbox#answer_19118

在 MATLAB Online 中打开

It's seeing I as the sqrt(-1). What is your goal by doing it this way?

why not just

syms A I
A+I

?

The other easiest workaround would just be to use a different letter, J, or II.

sym('A+II')

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Sean de Wolski 2011-8-18

Paulo: sym('A+I')

Walter Roberson 2011-8-18

MuPad allows variable names that are not valid MATLAB variable names, so as long as you are processing at the MuPAD level, you could, for example, replace I by _I and strip off the underscore later.

Or.... http://www.mathworks.com/help/toolbox/mupad/datatypes/DOM_IDENT.html#DOM_IDENT

Additionally, an arbitrary string of characters enclosed in ‘backticks’ `` is also an identifier. Examples: `x+y`, `a plus 1`. If the string of characters between the back ticks is a valid identifier already, this input form creates the same identifier as the one without the backticks.

Thus, you could substitute (say) `I ` (with a space) where you have the variable I in your input, and the only visible difference with normal typesetting turned on would be an extra space after the I .

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