Similar concept for “nilpotent” matrix but gives the identity and not 0

matricesnilpotencepermutations

Is there a term for a square matrix $E$ such that $E^k=I$ for some positive integer $k$?
To provide context: I was experimenting with permutation matrices and discovered that they satisfy the interesting property above. I have not proved/disproved this claim, I am looking for some hints and suspect that it has a nice name. Initially I thought it would be called "unipotent" (like how "nilpotent" is defined) but that's not it.

Best Answer

For $k = 2$ the term is involutory matrix - a matrix that is its own inverse. In general matrices such that that a power of them is the identity are called matrices of finite order.

You can think about it this way - the repeated application of such a matrix to a vector must ultimately give same the vector back since repeated applications eventually lead to multiplying by the identity. That means that for a matrix where $E^k = I$ all of the eigenvalues $\lambda_i$ must satisfy $\lambda_i^k =1$. I believe that condition defines the set of matrices with this property.

See also Is permutation matrix the only matrix for which $A^k = A$