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How to create a symbolic variable at special range.

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Samouil
Samouil on 20 Sep 2011
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Commented: Walter Roberson on 18 May 2020
I would like to create the symbolic variable 'x', which belongs to subspace [0. , 1.] of 'real 1d space'. I found that, "x=sym('x','real')" but it is not general as I would like "x=sym('x','[a, b]')", where 'a' and 'b' are real numbers with constrain ' a <= b '. Thanks.

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Answers (2)

Walter Roberson
Walter Roberson on 20 Sep 2011
Symbolic computations do not check constraints in some (common) operations, so although you could use assume() to add the constraint, there are more robust ways.
What you can do instead in MuPAD is to use Dom::Interval . This will have to be done at the MuPAD level, as this interface has not been exported to MATLAB. For example,
x = feval(symengine,'Dom::Interval', a, b);
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Walter Roberson
Walter Roberson on 18 May 2020
You would not do that by putting assumptions on t. You would do something like
symst
f(t) = exp(t)
fplot(f, [eps(realmin) 10*(1-eps)])
If it was especially important that you reach arbitrarily close to the limits without being at the limits, then use piecewise:
symst
f(t) = piecewise(0 < t & t < 10, exp(t));%不否则关闭意味着定义
fplot(f, [0 10])

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Xiangjie Yan
Xiangjie Yan on 18 May 2020
hi Samouil,
try
assume(x,'positive');
assume(1-x,'positive');
hope this can help :)

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