Using the least-squares method, we construct a new iterative procedure for finding solutions of an autonomous weakly nonlinear boundary-value problem in the critical case in the form of a generalized Fourier polynomial expansion.
Statement of the ProblemWe investigate the problem of the construction of solutions z.t; "/W z. ; "/ 2 C 1 OEa; b."/; z.t; / 2 C OE0; " 0 ; b. / 2 C OE0; " 0 ; of the autonomous boundary-value problem dz dt D Az C f C "Z.z; "/;`z. ; "/ D˛C "J.z. ; "/; "/;˛2 R m :We seek a solution of the Noetherian .m ¤ n/ problem (1) in a small neighborhood of theHere, A is a constant n n matrix, Z.z; "/ is a nonlinear vector function continuously differentiable with respect to the unknown z in a small neighborhood of a solution of the generating problem and continuously differentiable with respect to the small parameter " on the segment OE0; " 0 ;`z. ; "/ is a linear vector functional, J.z. ; "/; "/ is a nonlinear vector functional,`z. ; "/; J.z. ; "/; "/W C OEa; b."/ ! R m ; and the latter functional is continuously differentiable with respect to the unknown z and the small parameter " in a small neighborhood of a solution of the generating problem and on the segment OE0; " 0 : In the critical case .P Q ¤ 0/; under the condition P Q d f˛ `KOEf . /g D 0; problem (2) has the family of solutions [1, 2] z 0 .t; c r / D X r .t /c r CGOEf I˛.t /; c r 2 R r : Here, Q D`X. / is an m n matrix, rank Q D n 1 ; n n 1 D r; P Q is an m m orthoprojector matrix, P Q W R m ! N.Q /; X.t/ is the normal .X.a/ D I n / fundamental matrix of the homogeneous part of the differential system (2), X r .t/ D X.t/P Q r ; P Q r is an n r matrix composed of r linearly independent columns of the n n orthoprojector matrix, P Q W R n ! N.Q/; P Q d is a d m matrix composed of d D m n 1 linearly independent rows of the orthoprojector matrix P Q ; GOEf I˛.t / D X.t /Q C f˛ `KOEf . /g C KOEf .t/ is the generalized Green operator of problem (2), Q C is the Moore-Penrose pseudoinverse matrix [1],