function u = formalseries(x, n, a)
if nargin < 3
    a = 0;
end
if nargin < 2
    n = 12;
end
x = sym(x);

u = 0;
for i = 0:n
    tmp = strcat('a', num2str(i));
    v = sym(tmp);
    u = u + v*(x-a)^i;

end

