How to create a symbolic variable at special range.

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Samouil
Samouil 2011-9-20
评论： Walter Roberson 2020-5-18
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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回答（2 个）

Xiangjie Yan
Xiangjie Yan 2020-5-18
hi Samouil,
try
assume(x,'positive');
assume(1-x,'positive');
hope this can help :)
Walter Roberson
Walter Roberson 2011-9-20
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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tunahan yilmaz
tunahan yilmaz 2020-5-9
Also I am looking for this particular question which is defining a symbolic variable in an interval(range). I want to make a plot for f(t) = exp(t) for some range such as 0<t<10.
Walter Roberson
Walter Roberson 2020-5-18
You would not do that by putting assumptions on t. You would do something like
syms t
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:
syms t
f(t) = piecewise(0 < t & t < 10, exp(t)); %no otherwise close implies undefined
fplot(f, [0 10])

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