# What is Homotopy Continuation?

We briefly explain the ideas behind HomotopyContinuation.jl

The basic idea of homotopy continuation is as follows. Suppose that you have a system of polynomials with as many variables as polynomials

$$F(x)=\begin{bmatrix} f_1(x_1,\ldots,x_n) \\ \vdots\\ f_n(x_1,\ldots,x_n) \end{bmatrix}.$$

(for more polynomials than variables, see the guide on overdetermined tracking). We wish to find a zero of $F$ in $\mathbb{C}^n$. Suppose

$$G(x)=\begin{bmatrix} g_1(x_1,\ldots,x_n) \\ \vdots\\ g_n(x_1,\ldots,x_n) \end{bmatrix}$$

is another system with a known zero: $G(\zeta) = 0$. We connect $F$ and $G$ in the space of polynomial systems by a path $t\mapsto H(x,t)$ with $H(x,0) = F(x)$ and $H(x,1)=G(x)$ (the space of polynomial systems form a vector space, in which the notion of path is well-defined).

The idea in homotopy continuation is to approximately follow the solution path $x(t)$ defined by $H(x(t),t)=0$. For this, the path is discretized into time steps $t_0 = 0 < t_1 < \cdots < t_k = 1$. If the discretization is fine enough then the Newton’s operator of $H(x,t_1)$ applied to $\zeta$ converges to a zero of $H(x,t_1)$. Then, if $t_2-t_1$ is small enough and if $\Vert \zeta_i-\xi\Vert$ is small enough for some $i\geq 0$, the iterate $\zeta_i$ converges to a zero of $H(x,t_2)$. We may repeat the procedure for $H(x,t_2)$ and starting with $\zeta_i$. Inductively, we find an approximate zero of $H(x,t_j)$ for all $j$. In the end, we will find an approximate zero for the system $F(x)=H(x,t_k)$.

Most implementations of homotopy continuation, including Bertini and HomotopyContinuation.jl, use heuristics for setting the step-sizes $t_{j+1}-t_j$ and number of Newton iterations.

For instance, the homotopy could be the straight-line homotopy $tG + (1-t)F$. The default choice of HomotopyContinuation.jl is a slightly modified straight-line homotopy. If your system has parameters, you can also make a parameter homotopy. You can even make up your own custom homotopy. For square systems is that we can automatically generate a start system $G$. If you use the basic solve command the following starting system is constructed:

$$G(x_1,\ldots,x_n) = \begin{pmatrix} x_1^{d_1} - a_1 \\ \vdots \\ x_n^{d_n} - a_n\end{pmatrix},$$

where the $a_i$ are random numbers and $d_i$ is the degree of $f_i$. There are $d_1\cdots d_n$ many solutions to this system, which are easy to write down. A theorem by Bézout says that a system whose $i$-th entry is a polynomial of degree $d_i$ has at most $d_1\cdots d_n$ solutions (if not infinitely many). Hence, tracking all $d_1 \cdots d_n$ solutions of $G$ to $F$ we are can find all solutions of $f$. Such a homotopy is called a total degree homotopy.

(For researchers interested in mathematical proofs: although the outcome of the homotopy continuation algorithms are numerical approximation of the true zeros of $F$, there is a way of rigorously certifying if a true zero is closeby or not. We refer to the paper by Hauenstein and Sottile and their algorithm alpha-Certified.)