, Graeffe's method
is an algorithm for finding multiple roots of a polynomial. It was developed independently by Karl Heinrich Gräffe
, and Lobachevsky
. The method separates the roots of a polynomial by squaring them repeatedly, and uses the Vieta relations
in order to approximate the roots.
The roots of (when viewed as a polynomial in the variable ) are . We have squared the roots of our original polynomial . Iterating this procedure several times separates the roots with respect to their magnitudes. Repeating k times gives a polynomial in the variable of degree n. Write this polynomial as
with roots .
Classical Graeffe's method
Next the Vieta relations are used
If the roots are sufficiently separated, say by a factor , , then the iterated powers of the roots are separated by the factor , which quickly becomes very big.
The coefficients of the iterated polynomial can then be approximated by their leading term,
- and so on.
Finally logarithms are used in order to find the roots of the original polynomial.
Graeffe's method works best for polynomials with simple real roots, though it can be adapted for polynomials with complex roots and double roots. This method is problematic since the coefficients of the iterated polynomials span very quickly many orders of magnitude, which implies serious numerical errors. One second concern is that many different polynomials lead to the same Graeffe iterates.
Tangential Graeffe method
If is an „algebraic infinitesimal“ with , then the polynomial
has roots , with powers
Thus the value of
is easily obtained as fraction
This kind of computation with infinitesimals is easy to implement analogous to the computation with complex numbers. If one assumes complex coordinates or an initial shift by some randomly chosen complex number, then all roots of the polynomial will be distinct and consequently recoverable with the iteration.
Every polynomial can be scaled in domain and range such that in the result the first and the last coefficient have size one and all intermediate coefficients a size smaller then one. This implies that all roots are located in a ring between the radii 1/2 and 2. If the size of the inner coefficiens is bounded by M, then the size of the inner coefficients after one stage of the Graeffe iteration is bounded by . After k stages one gets the bound for the inner coefficients If the initial coefficients
To overcome the limit posed by the growth of the powers, Malajovich/Zubelli propose to represent coefficients and intermediate results in the kth stage of the algorithm by a scaled polar form