It is defined as taking a fixed base in any two vector spaces and taking the sum of these two bases as the base of the direct sum of vector spaces, then the direct sum of linear transformations in these two spaces can be expressed as the direct sum of two matrices. Direct sum, a mathematical term, refers to the special relationship between two subspaces in linear space. Chinese name: He Zhi, mbth: directsum. Application field: linear algebra, subject: mathematics.