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In linear algebra, a semi-orthogonal matrix is a non-square matrix with real entries where: if the number of columns exceeds the number of rows, then the rows are orthonormal vectors; but if the number of rows exceeds the number of columns, then the columns are orthonormal vectors.
Let be an semi-orthogonal matrix.
Consider the matrix whose columns are orthonormal: Here, its columns are orthonormal. Therefore, it is semi-orthogonal, which is confirmed by:
Consider the matrix whose rows are orthonormal: Here, its rows are orthonormal. Therefore, it is semi-orthogonal, which is confirmed by:
The following matrix has orthogonal, but not orthonormal, columns and is therefore not semi-orthogonal: The calculation confirms this:
If a matrix is tall or square (), its semi-orthogonality implies . For any vector , preserves its norm: If a matrix is short (), it preserves the norm of vectors in its row space.
If , then the columns of are linearly independent, so the rank of must be . If , then the rows of are linearly independent, so the rank of must be . In both cases, the matrix has full rank.
The statement is that a real matrix is semi-orthogonal if and only if all of its non-zero singular values are 1.