TITLE: Asymptotic series for roots of polynomials
QUESTION [5 upvotes]: Let $f(z) = z + z^2 + z^3$.   Then for large $n$, $f(z) = n$ has a real solution near $n^{1/3}$, which we call $r(n)$.  This appears to have an asymptotic series in descending powers of $n^{1/3}$, which begins
$$ r(n) = n^{1/3} - {1 \over 3} - {2 \over 9} n^{-1/3} + {7 \over 81} n^{-2/3} + O(n^{-1}). $$
In order to derive this series I use a method of undetermined coefficients.  If we assume such a series exists, of the form
$$ r(n) = An^{1/3} + B + Cn^{-1/3} + Dn^{-2/3} + O(n^{-1}), $$
then applying $f$ to both sides gives
$$ n = A^3 n + (3 A^2 B + A^2) n^{2/3} + (3 A^2 C + 3 A B^2 + A + 2AB)n^{1/3} + (B^2 + 2AC + 3A^2 D + B + B^3 + 6 ABC + O(n^{-1/3}). $$
Thus we have $A^3 = 1$ and so $A = 1$; $3A^2 B + A^2 = 0$ and so $B = -1/3$; similarly $C = -2/9, D = 7/81$.
More generally, let $f$ be a polynomial with leading term $Ax^k$.  Then the largest real root of $f$ appears to have an asymptotic series of the form
$$ A^{-1/k} n^{1/k} + c_0 + c_1 n^{-1/k} + c_2 n^{-2/k} + \cdots $$
which can be computed by a similar method to the one I gave above.  
My question, which I would like an answer to in order to make a minor point in my dissertation: how can I prove that the solutions of equations of the form $f(z) = n$ have such series?

REPLY [4 votes]: If you have a good initial guess (such as your case), then you don't need either undetermined coefficients or Newton's method (although they both work), you can in fact get a procedure which will give you as many coefficients as you want.  Harald's first step is excellent, so we'll start from that.  Given $F(\epsilon,w) = \epsilon^2w+\epsilon w^2+w^3-1 = 0$, we can derive a differential equation for $w(\epsilon)$.  With the help of Maple's gfun[algeqtodiffeq], giving
$$ 9+ \left( 6{\epsilon}^{5}+7{\epsilon}^{2} \right) w ( \epsilon ) + \left( 27-6{\epsilon}^{6}-7{\epsilon}^{
3} \right) {\frac {d}{d\epsilon}}w(\epsilon) + \left( -3{\epsilon}^{7}-14{\epsilon}^{4}-27\epsilon \right) {\frac {d^{2}}{d{\epsilon}^{2}}}w ( \epsilon ) $$
with initial conditions $w (0) =1,w^{'''}(0) =-4/9 $ [the DE is singular at $\epsilon=0$ so the initial conditions are non-standard).  Nevertheless, from there one can continue and use gfun[diffeqtorec] to get a recurrence for the coefficients of the series.  The result is
$$ \left( 6-3n-3{n}^{2} \right) u(n) + \left( -98-77n-14{n}^{2} \right) u (n+3) + \left( -
648-27{n}^{2}-270n \right) u (n+6 ) $$
with starting conditions $u(0) =1, u(1) =-1/3,u (2) =-2/9,u (3) ={\frac {7}{81}},u(4) =0,u(5) ={\frac {14}{729}}$.  These are sufficient to allow you to 'unwind' the recurrence as much as you want.  
Bjorn's answer gives the underlying 'analytic' answer that justifies that the above gives a convergent result.  Actually, the procedure above works in other cases as well, but then the result is only valid in a sector, and computing the size of that sector can be fiendishly difficult.