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More on mortality. (English) Zbl 0702.15017

There are three questions with remarks in this note.
1. Does there exist an algorithm which decides for any finite set of \(2\times 2\) matrices with integer entries whether the set is mortal?
2. Does there exist an algorithm which decides for any finite set P of nonsingular \(2\times 2\) matrices with integer entries whether there is a product that is formed with members of P that is equal to a matrix C such that \(C_{21}=C_{22}?\)
3. Does there exist an algorithm which decides for any finite set P of nonsingular lower triangular \(2\times 2\) matrices with integer entries whether there is a product that is formed with members of P that is equal to a matrix C such that \(C_{21}=C_{22}\)?
Reviewer: Yueh-er Kuo

MSC:

15B57 Hermitian, skew-Hermitian, and related matrices
65F30 Other matrix algorithms (MSC2010)
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