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different results in R2012x and R2013x of function eig for matrices of class sym

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Nanny Wermuth
Nanny Wermuth on 9 Jan 2014
Closed: MATLAB Answer Bot on 20 Aug 2021
whos Name Size Bytes Class Attributes
a 1x1 112 sym
mat 5x5 112 sym
a =
a
mat =
[ 1, a^2, a^2, a^2, a]
[ a^2, 1, a^2, a^2, a]
[ a^2, a^2, 1, a^2, a]
[ a^2, a^2, a^2, 1, a]
[ a, a, a, a, 1]
The result of
simplify(prod(eig(mat)))
is up to MATLAB_R2012b
(a^2 - 1)^4
which is the expected result.
Since MATLAB_R2013a the outcome is
(a^2 - 1)^2
How can it be fixed in R2013x?
  1 Comment
Roger Stafford
Roger Stafford on 9 Jan 2014
Yes, (1-a^2)^4 is the correct result. Even my ancient twenty-year-old version of Symbolic Toolbox gets it right. If R2013a gets (1-a^2)^2, that is surely a bug and should be reported to Mathworks.

Answers (1)

Sean de Wolski
Sean de Wolski on 9 Jan 2014
I can reproduce this. Please contact support.

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