- U(j) is the j.th element of the vector U.
- U(1, j) is the j.th element of the first row of U.
- U(:, j) is the j.th element of all rows of U.
As long as U is a row vector, these three terms create the same answer.
The indexing is explained exhaustively in the "Getting Started" chapters of the documentation. It is obligatory to read at least these chapters, when you want to work with such a powerful tool as Matlab.
