For analysis of the sum σ$_{c}$ ( ⃗ k ) near the point ⃗ k = ⃗ k$_{0}$ we note that because of the evident relations the value of σ$_{c}$ ( ⃗ k$_{0}$ ) = 0. The next terms of the expansion σ$_{c}$ ( ⃗ k ) over the vector components ⃗ q = ⃗ k − ⃗ k$_{0}$ can be written easily, They comprise both real terms proportional to q 2 x − q 2 y and imaginary terms like q$_{x}$q$_{y}$ . After simple algebra, these terms are presented via the sum D introduced above and one more convergent sum of similar structure Finally, in the quadratic approximation over ⃗ q = ⃗ k − ⃗ k$_{0}$ near the point ⃗ k$_{0}$ of the type (1,1) one can present the sum as This function, in contrast with σ ( ⃗ k ) near the same point ⃗ k$_{0}$ , is anisotropic. However, expressions for the frequencies contain σ ( ⃗ k ) and σ$_{c}$ ( ⃗ k ) only in the combination σ $^{2}$( ⃗ k ) − 9 | σ$_{c}$ ( ⃗ k ) | $^{2}$, therefore for presentation of the spectrum of collective modes accurate to | ⃗ q | 2 it is not essential to take σ$_{c}$ ( ⃗ k ) into account, and to the accuracy of q 2 the spectrum near the point ⃗ k$_{0}$ of the type (1,1) is radially symmetric over components of the vector ⃗ q . The point with ⃗ k$_{0}$ of the type (1,0) including the points like ⃗ k$_{0}$ = ± π⃗ e$_{x}$/a or ⃗ k$_{0}$ = ± π⃗ e$_{y}$/a , possess lower (biaxial) symmetry than the points ⃗ k = 0 or points of the type (1,1). The rather complicated study in the vicinity of this point ( ⃗ q = ⃗ k − ⃗ k$_{0}$ is small) shows the analytical dependence for the components of ⃗ q , in this case for the components of ⃗ q parallel and perpendicular to ⃗ k$_{0}$ σ ( ⃗ k ) − σ ( ⃗ k$_{0}$ ) = 1 2 a $^{2}$( D$_{|}$$_{|}$q 2 || − D$_{⊥}$q 2 $_{⊥}$) , σ$_{c}$ ( ⃗ k ) = σ$_{c}$ ( ⃗ k$_{0}$ ) + i ( D$_{1}$ − D ) a $^{2}$q$_{x}$ q$_{y}$ + 1 4 D$_{1}$a $^{2}$( q 2 x − q 2 $_{y}$) . D$_{1}$ = − ∑ ( l 2 x − l 2 $_{y}$) $^{2}$( − 1) l$_{x}$ + $^{l$_{y}$}$/ ( l 2 x + l 2 y ) − 5 / 2 ∼ = 3 . 31891 . σ$_{c}$ ( ⃗ k ) = 2 iq$_{x}$q$_{y}$ ∑ l 2 $_{x}$l 2 y ( l 2 x + l 2 y ) 5 / $_{2}$( − 1) l$_{x}$ + $^{l$_{y}$}$+ + 1 2 ( q 2 x − q 2 y ) ∑ l 2 $_{y}$( l 2 x − l 2 $_{y}$) ( l 2 x + l 2 y ) 5 / $_{2}$( − 1) l$_{x}$ + $^{l$_{y}$}$. (28) ∑ ⃗ l =0 ( l 2 x − l 2 $_{y}$) · ( l 2 x + l 2 $_{y}$) − 5 / $^{2}$( − 1) l$_{x}$ + l$_{y}$ = 0 , 29