function [mat] = A(t,y,n,dt)
% Construct the matrix A 
for j = 1:n,
  q = -1; % Same problem as in Lecture 16
  % q = -t(j)*y(j);  % Non-linear problem
  mat(j,j) = -(2+(dt^2)*q);
end
diagonal_plus_one_vec = ones(n-1,1);
diagonal_minus_one_vec = ones(n-1,1);
  
mat = mat + diag(diagonal_plus_one_vec,1) + diag(diagonal_minus_one_vec,-1);