How to create a symbolic variable at special range.

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.

Answer 1

[Xiangjie Yan]

hi Samouil,

try

assume(x,'positive');
assume(1-x,'positive');

hope this can help :)

Answer 2

[Walter Roberson]

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);

[Samouil]

I try it, but I think dose not work.

Think that you have the parameter 'x,y,z' as '0.<=x,y,z<=1.', and the function :

a = y/(1-x);

b = z/(1-x);

f = simplify(a/(1-b))

the result is -y/(x+z-1).

If I do that 'x = feval(symengine,'Dom::Interval', a, b);' I get as result 'Dom::Interval(0, Inf) union Dom::Interval(-Inf, 0)' ????

[tunahan yilmaz]

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]

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])