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$$xbox(i,j) = I_{[i=n/4,\;3n/4,\;j;~ n/4 \leq j \leq 3n/4]} + $$$$ I_{[n/4 \leq i \leq 3n/4;~ j=n/4,\;3n/4,\;i]}$$
data(xbox)
A 128 \(\times\) 128 matrix.
Bruce, A., and H.-Y. Gao (1996) Applied Wavelet Analysis with S-PLUS, Springer: New York.