Intervals in the real line can be specified with the interval element. It expects exactly two children elements, which encode the end points. The default is closed. A vector element constructs a vector whose components are given in order by its children. By convention, in MathML vectors are column vectors for matrix multiplication.
Matrices actually require two elements, matrix and matrixrow .
Although matrix rows are a little odd to single out from a mathematical viewpoint, they are necessary crutch for encoding matrices. A matrix element expects any number of children, but they have to all be matrixrow elements. The children of the matrixrow elements represent the individual entries in the matrix. All matrix rows should have the same number of elements. There are around 50 empty operator elements in content markup, which represent commonly used functions and operators.