Let A be a real, invertible n &#x00D7;<!-- × --> n matrix. I am interested in finding th

Armeninilu

Armeninilu

Answered question

2022-06-21

Let A be a real, invertible n × n matrix. I am interested in finding the vectors x R n that solve the following equation:
x = A tanh ( x )
where the tanh is applied element-wise. More generally, we can consider other kinds of non-linearities instead of the tanh (but always applied element-wise).
Is there a generic approach to studying the solutions of these type of equations? Probably exploiting the eigen decomposition of A?
I added the tag "reference-request" in case someone can suggest relevant references to the literature.

Answer & Explanation

Ethen Valentine

Ethen Valentine

Beginner2022-06-22Added 15 answers

In the 2D case, the equation takes the form
{ x = a f ( x ) + b f ( y ) , y = c f ( x ) + d f ( y )
and after elimintation of y, we get a univariate nonlinear equation
x a f ( x ) b = f ( c f ( x ) + d b ( x a f ( x ) ) .
We don't see any particular simplification nor connection with the Eigenvalues.
I have seen numerical cases with four distinct positive solutions.

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