I am looking at a formula, simplified here, which says that peaks occur at wavelengths: \lambda_{ij}=...(i^{2}+j^{2})... "where i, j correspond to the orders of the 2D wave vector"
The peak wavelengths are at \lambda_{10},\lambda_{11},\lambda_{20}
(i.e. where (i^{2}+j^{2}) terms are...