Walter Kohn and Norman Rostoker derived this same theory using the Kohn variational method. It is called the Korringa–Kohn–Rostoker method (KKR method) for band theory calculations. Ewald derived a mathematically sophisticated summation process that makes it possible to calculate the structure constants, A l m , l ′ m ′ ( E , k ) . The energy eigenvalues of the periodic solid for a particular k , E b ( k ) , are the roots of the equation det M ( E , k ) = 0 . The eigenfunctions are found by solving for the c l , m ( E , k ) with E = E b ( k ) . The dimension of these matrix equations is technically infinite, but by ignoring all contributions that correspond to an angular momentum quantum number l greater than l max , they have dimension ( l max + 1 ) 2 . The justification for this approximation is that the matrix elements of the t-matrix t l m , l ′ m ′ are very small when l and l ′ are greater than l max , and the elements of the inverse matrix m l m , l ′ m ′ are very large.