What's the meaning of and ?

One of the image compression method is using SVD (Singular Value Decom- position). It treats an image as an m x n matrix A, and performs SVD on A Assume m > n, where > i +1 for-1, 2, . .. n-1. The compression process only keeps the k largest singular values and their corresponding singular vectors Figure 1 shows an example. The original picture is of size 267x189, as shown in 1.(a). After the compression process performs SVD, the new image only uses 50 singular values and singular vectors to reconstruct the image. The compressed image is shown in 1.(b). The compression ratio is 267 >