Kroneker delta

In equation 1, suffixes range from 1 to 3 and indicate the Cartesian coordinate axes along which the component is measured. Summation from 1 to 3 over repeated suffixes is implied, and a subscript comma indicates partial differentiation with respect to the indicated spatial coordinate. The Kroneker delta is Sy. 

Here 6a,fg are the surface angular coordinates and a/jis the Kroneker delta). In the absence of wetting interactions between the film and fhe subsfrate, such a surface is thermodynamically unstable and exhibits spontaneous formation of pyramidal faceted" structures that coarsen in time [10]. However, as we show below, the presence of wetting interactions can suppress this instability, or qualitatively change it, so that it would lead to the formation of spatially-regular arrays of islands. 

We seek an answer where is a diagonal matrix with all off-diagonal elements equal to zero, which is accomplished by the use of the Kroneker delta = o) = 0(p,/7 a). Then... 

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