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Remarks on the Belitskii–Lyubich and discrete Markus–Yamabe conjectures

  • Frédéric Sart EMAIL logo

Abstract

In this paper we prove the Belitskii–Lyubich conjecture for triangular and gradient mappings. These results resemble those obtained for the discrete Markus–Yamabe conjecture. However, the proofs are quite different, which sheds new light on the subject. We complete the picture by showing that the general versions of the two conjectures can be turned into theorems at little cost, simply by relaxing their spectral condition.

A Appendix

For completeness we also give the proofs of the following two lemmas, which seem to belong to mathematical folklore. Let A 1 , , A p , B be n × n matrices such that for all i , j , k , | ( A k ) i j | B i j .

Lemma 3.

For all i , j , p , | ( A 1 A p ) i j | ( B p ) i j .

Proof (by induction on p).

It is true for p = 1. Suppose it holds for p. Then

| ( A 1 A p A p + 1 ) i j | = | k = 1 n ( A 1 A p ) i k ( A p + 1 ) k j | (product definition)
k = 1 n | ( A 1 A p ) i k | | ( A p + 1 ) k j | (triangle inequality)
k = 1 n ( B p ) i k B k j = ( B p + 1 ) i j (induction hypothesis) .

Thus it holds for p + 1 and so it is true for all p. ∎

Lemma 4.

For all p, A 1 A p B p .

Proof.

We have

A 1 A p = max i j = 1 n | ( A 1 A p ) i j | (  definition)
max i j = 1 n ( B p ) i j (Lemma 3)
= B p (  definition)

as desired. ∎

Acknowledgements

I thank the referee for suggesting changes that greatly improved the readability of the paper.

References

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Received: 2021-12-04
Revised: 2022-05-22
Accepted: 2022-05-28
Published Online: 2022-10-26
Published in Print: 2023-06-01

© 2022 Walter de Gruyter GmbH, Berlin/Boston

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