Wikipedia:Reference desk/Archives/Mathematics/2023 July 10

= July 10 =

Term for matrix property
Is there a specific term for a matrix that can be raised to some power to give the identity matrix? I started to tell someone that if you can raise a matrix to some power and get zero it's nilpotent, and if you can raise it to some power and get the identity it's... but realized I don't know if there's a term for it. It's clearly not idempotent, and unipotent is slightly closer but also not right. The involutory matrices would be a subset. Should I say "a root of the identity" by analogy with the roots of unity? --Amble (talk) 14:32, 10 July 2023 (UTC)


 * Or you could even just say "root of unity". :-) –jacobolus (t) 16:27, 10 July 2023 (UTC)
 * Yes, I suppose that must be correct. I had been thinking there should be an adjective ___potent, analogous to nilpotent and idempotent, but perhaps there isn't one. This stackoverflow asks a similar question in slightly different context, and points to more or less the same answer. I think I'm happy with "roots of the identity matrix" (since in this case we were already talking about "roots of unity" as elements of the matrix). --Amble (talk) 17:12, 10 July 2023 (UTC)
 * Apparently the terms is "matrix of finite order", see for example the journal article (via JSTOR) A Classification of Matrices of Finite Order over C, R and Q. The same expression is used in group theory with 'matrix' replaced by 'element'. --RDBury (talk) 18:56, 10 July 2023 (UTC)
 * Yes, that makes sense, coming from a group theory point of view (although "order" can have other meanings for matrices!). Thanks! --Amble (talk) 19:09, 10 July 2023 (UTC)
 * The term is also used for semigroups (see ), and the $$n\times n$$ matrices over a ring form a semigroup under multiplication. --Lambiam 19:16, 10 July 2023 (UTC)