% ++--++--++-- series
AR(1) = 1;
AR(2) = 1;
for m = 3:nc,
 AR(m) = -AR(m-2);
end;

% imaginary part in alternating parity
parity = 1;
for m=1:nc,
 AI(m) = parity*AR(m);
 parity = -parity;
end;

A = AR + i*AI;

% spin wave rotation 
% twist = 8;
% damax = 2*pi/twist;
% phi(1) = 0;
% for m = 1:nc,
%  BR(m) = AR(m)*cos(phi(m))-AI(m)*sin(phi(m));
%  BI(m) = AR(m)*sin(phi(m))+AI(m)*cos(phi(m));
%  if rand(1) > 0.5
%   phi(m+1)=phi(m)+damax/2.;
%  else
%   phi(m+1)=phi(m)-damax/2.;
%  end;
% end;
% A = BR + i*BI;





