It is well known in the theory of elastic shells that a first order approximation using the shell thickness as an expansion parameter leads to the membrane theory of shells. The membrane equations have as solutions the generalized analytic functions. These functions have been exhaustively studied by Ilya N. Vekua [6], [7] and his students. R.P. Gilbert and J. Hile [3] introduced an extension of these systems to include elliptic systems of 2n equations in the plane and named the solutions of these systems generalized hyperanalytic functions.It is shown in this paper that the next order approximation to the shell, which permits, moreover, the introduction of bending, may be described in terms of the generalized hyperanalytic functions. It is strongly suspected that the higher order approximations may also be described in terms of corresponding hypercomplex systems.
Notation and formulationIn this work we follow the notation used by Dikmen [1]; we view a shell as a three dimensional body, which we try to reduce to two dimensional consideration by introducing a suitable reference surface. A set of curvilinear coordinates 0 i, (i = 1, 2, 3) are chosen so that a reference surface within the shell may be represented by 0 3 = 0. For purposes of defining our notation let a be a surface embedded in R 3 which we represent in the form r = r(0~). The vectors a~ .'=r~ = (ar)/(O0 ~) are the base vectors. The first fundamental form of the surface is given by a~p d0~d0 a where the a~a.-=a~ "ap are the covariant components of the metric tensor. If the .4 ~p are the cofactors of the a~p we define the contravariant metric tensor as a~a:=.4~a/a where a is the determinant of a~p. The second fundamental form of the surface is defined through b~ a dO ~ d0a:= a3 • a~,a dO ~ d0 a = -dr" da3, with a3 = (al × a2)/([al )< a2l). The Christoffel symbols are given as usual by . i F~a ~ .= ~(a~,a + ap~,~ -a~a,~ ), and covariant differentiation is defined by , r'~ Tx. (1.1) T~I := T~ -The position vector of a generic point on the shell at time t is given by x = x(O ~, ~, t) where (0 l, 0 2) ~ i'~ is a point on the reference surface and ~ [0, h], in particular x(O ~, O, t)= r(O~, t). The methods of Dikmen [1] and