negative zero as eigenvalue

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Suma
Suma 2014-4-10
评论: Suma 2014-4-12
Hello,
Why do I get negative zero as eigenvalue?
ans =
0.2615 0.1950 - 0.3938i
0.1950 + 0.3938i 0.7384
>> eig(ans)
ans =
-0.0000
1.0000
thanks
  2 个评论
abhijit kulkarni
abhijit kulkarni 2014-4-10
I think that negative zero implies that it is not treated as positive integer.
Refer: The IEEE 754 standard for floating-point arithmetic (presently used by most computers and programming languages that support floating point numbers) requires both +0 and −0. Real arithmetic with signed zeros can be considered as a variant of the extended real number line such that 1/−0 = −∞ and 1/+0 = +∞; division is only undefined for ±0/±0 and ±∞/±∞.
Negatively signed zero echoes the mathematical analysis concept of approaching 0 from below as a one-sided limit, which may be denoted by x → 0−, x → 0−, or x → ↑0. The notation "−0" may be used informally to denote a small negative number that has been rounded to zero. The concept of negative zero also has some theoretical applications in statistical mechanics and other disciplines.
It is claimed that the inclusion of signed zero in IEEE 754 makes it much easier to achieve numerical accuracy in some critical problems,[1]
Suma
Suma 2014-4-10
Thanks,
Then I have a big problem. I do want high accuracy but then on the other hand it would lead me to an imaginary output further down the line in my work. Is is alright to convert such negative zero value to positive one? How can I convert these negative zero eigenvalue to positive zero in matlab?
Looking forward for suggestion
thanks

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采纳的回答

Mischa Kim
Mischa Kim 2014-4-10
Suma, it's not really zero. Set the display format to long fixed decimal:
format long
[v, d] = eig(a)
v =
0.381333979059441 - 0.770099081813373i 0.226935196532721 - 0.458292719972234i
-0.511401799571426 + 0.000000000000000i 0.859341724458383 + 0.000000000000000i
d =
-0.000011841043894 0
0 0.999911841043894
  5 个评论
Suma
Suma 2014-4-10
编辑:Suma 2014-4-10
could you elaborate:
e-16i = i*10^(-16)
where does "i" come from?
thank you
Mischa Kim
Mischa Kim 2014-4-10
i is the imaginary unit. Since the matrix is complex you can expect the eigenvectors to be complex as well.

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更多回答(2 个)

Suma
Suma 2014-4-10
Hi,
Could it be that actually it is zero but due to computational approximation required it is showing -0.000011841043894 for example or is it reality?
  1 个评论
lvn
lvn 2014-4-10
编辑:lvn 2014-4-10
No, Just verified with another program: {0.999912, -0.000011841}

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Suma
Suma 2014-4-10
编辑:Suma 2014-4-10
if I take the value as it is as negative zero, I obtain a result as 0.83149-5.8003e-16i
i even don't know what it means- there is minus in between 0.83149 and 5.8003 and exp to the power of i. it is small value or large?
also, how do I approximate the precision so that I get zero eigenvalue for case of negative zero? how do i replace?
  3 个评论
Alberto
Alberto 2014-4-10
what you have there is a complex number in the form a+b*i, where a=0.83149 and b=-5.8003e-16 (see the 'i' in the end?). The norm is like a, because b has a very small value (a^2+b^2 ~ a^2)
Suma
Suma 2014-4-12
i accepted an answer but want to thanks all

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