In this paper we study singular values of a matrix whose one entry varies while all other entries are prescribed. In particular, we find the possible pth singular value of such a matrix, and we define explicitly the unknown entry such that the completed matrix has the minimal possible pth singular value. This in turn determines possible pth singular value of a matrix under rank one perturbation. Moreover, we determine the possible value of pth singular value of a partially prescribed matrix whose set of unknown entries has a form of a Young diagram. In particular, we give a fast algorithm for defining the completion that minimizes the pth singular value of such matrix.
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