function [n, dxi, deta] = plate_fem_shape(xi, eta)
% PLATE_FEM_SHAPE  Bilinear shape functions of the four-node element.
%
%   [n, dxi, deta] = plate_fem_shape(xi, eta) at a point of the reference
%   square [-1, 1]^2: the four functions, ordered anticlockwise from the
%   corner (-1, -1), and their derivatives with respect to xi and eta.

n = 0.25 * [(1 - xi) * (1 - eta), (1 + xi) * (1 - eta), (1 + xi) * (1 + eta), (1 - xi) * (1 + eta)];
dxi = 0.25 * [-(1 - eta), 1 - eta, 1 + eta, -(1 + eta)];
deta = 0.25 * [-(1 - xi), -(1 + xi), 1 + xi, 1 - xi];
end