Exact Description of D-Branes in K-Matrix Theory

We summarize how to describe D-branes in a matrix theory based on unstable D-instantons, which we call K-matrix theory, and explicitly show that D-branes can be constructed as bound states of infinitely many unstable D-instantons. We examine the fluctuations around Dp-brane solutions in the matrix theory and show that they correctly reproduce fields on the Dp-brane world-volume. Plugging them into the action of the matrix theory, we precisely obtain the D p -brane action as the effective action of the fluctuations.