function Polygon(a0,b0,radius,n_sides)
%
% displaying a Polygon with n_sides.
% A circle is treated as a single case of a polygon,
% it is a polygon with 360 sides.
%
% a0, b0 - its center,
% radius - the radius enclosing the polygon,
% n_sides - the number of sides.
%
% (x_first,y_first) - the starting vertex of a polygon.
%
x_first=a0+radius;
y_first=b0;
for j=1:n_sides
  % (x_next,y_next) - the next vertex of a polygon.
  x_next=a0+radius*cos(2*j*pi/n_sides);
  y_next=b0+radius*sin(2*j*pi/n_sides);
  line([x_first x_next], [y_first y_next]);
%
  if  n_sides<6
    % displaying the vertice number.
    text(x_first,y_first,num2str(j))
 end
  x_first=x_next;
  y_first=y_next;
end % j