Then defining F ( x ) = ∑ ∑ n = 1 N β β n e − − χ χ n x + 1 2 π π ∫ ∫ R r ( k ) e i k x d k , where the β β n are non-zero constants, solving the GLM equation K ( x , y ) + F ( x + y ) + ∫ ∫ x ∞ ∞ K ( x , z ) F ( z + y ) d z = 0 for K allows the potential to be recovered using the formula u ( x ) = − − 2 d d x K ( x , x ) .