“…the second order linear equation with constant coefficients ψ xx − k 2 4 ψ = 0 (7.30) Ψ 2 = C 1 e k 2 x + C 2 e − k 2 x = C 0 cosh k 2 (x − x 0 ), (7.31) ψ 1 (x) = Ψ 2 (x + a), ψ 2 (x) = Ψ 2 (x − a), a arbitrary, (7.32) Substituting (7.29) into (7.17) and eliminating any derivative of (ψ 1 , ψ 2 ) of order higher than or equal to two in x results into a polynomial in the two variables ψ 1,x /ψ 1 , ψ 2,x /ψ 2 . Before identifying it to the null polynomial, one must take account of the first integral µ 0 , the ratio of two constant Wronskians 33) which splits the polynomial of two variables into the sum of two polynomials in one variable : are just two different representations [34] of a solution of (7.17) depending on two arbitrary constants (µ 0 , x 0 ) : with this simple assumption, we have obtained the general solution u −2 = v = 1 2β…”