The real() command outputs a complex number (doesn't appear to change it).

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Patrick Coelen
Patrick Coelen 2023-1-20
编辑： Paul 2023-1-21
I want to evaluate the real part of some eigenvalues, but real(eigen) just returns eigen (isreal(real(eigen) returns 0).
  2 个评论
Stephen23
Stephen23 2023-1-21
编辑：Stephen23 2023-1-21
Why do you imagine that ISREAL() can be applied to symbolic variables? It is not listed as a function of the symbolic toolbox:
https://www.mathworks.com/help/symbolic/referencelist.html
and the ISREAL() documentation clearly states that it checks "whether array uses complex storage", i.e. it checks for something that is purely a characteristic of numeric arrays and has nothing to do with symbolic mathematics. See:
https://www.mathworks.com/help/matlab/ref/isreal.html
In contrast, REAL() is a function of the symbolic toolbox, and works exactly as described:
https://www.mathworks.com/help/symbolic/real.html
Paul
Paul 2023-1-21
编辑：Paul 2023-1-21
Ran in:
Just because a function is not listed as a function of the symbolic toolbox in its documentation does not mean that function can't be applied to a sym variable. We can further break this situation down into two cases.
First, functions that are not methods of the syms class, but work fine on sym objects because the underlying code in the function all works on sym. The function dot is a good example
x = sym('x',[1 3])
x = 
dot(x,x)
ans = 
Second, some methods of the sym class (at least one, I can't say for sure more than one) are just not documented. For example, exp is a sym method, but is (shockingly, IMO) not listed as a function of the symbolic toolbox on that linked page.
exp(x)
ans = 
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采纳的回答

Torsten
Torsten 2023-1-20
isreal(real(double(eigen(t))))
instead of
isreal(real(eigen(t)))

更多回答（1 个）

Paul
Paul 2023-1-21
Ran in:
In syms world, use isAlways to determine if an expression or variable is real, etc.
syms x
isAlways(in(x,'real'))
Warning: Unable to prove 'in(x, 'real')'.
ans = logical
0
assume(x,'real')
isAlways(in(x,'real'))
ans = logical
1

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