 Substituting in , one obtains in the first order

where quantities "aac" constitute a matrix that is inverse to matrix "aac".The solution of has the form

where "la", "ma", ρ, are arbitrary functions of coordinates "x", "y" bound by condition derived from .To find higher terms of this decomposition, it is convenient to write the matrix of required quantities γ"ab" in the form

where the symbol ~ means matrix transposition.