When is a matrix a sum of involutions or tripotents?

We give the necessary and sufficient conditions for an n × n matrix over an integral domain to be a sum of involutions and, respectively, a sum of tripotents. We determine the integral domains over which every n × n matrix is a sum of involutions and, respectively, a sum of tripotents. We further determine the commutative reduced rings over which every n × n matrix is a sum of two tripotents. [ABSTRACT FROM AUTHOR]