# Total Degree Homotopy

The total degree start system

Total degree is a particular choice of start system for homotopy continuation.

It is the simplest start system for a system

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

The total degree start system of $F$ is

$$G(x_1,\ldots,x_n) = \begin{bmatrix} x_1^{d_1} - a_1 \\ \vdots \\ x_n^{d_n} - a_n\end{bmatrix}, \text{ where } d_i = \text{deg}(F_i),$$

and where the $a_i$ are random numbers. 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.

Here is how it works:

julia> using HomotopyContinuation
julia> @var x y;
julia> f = System([x^2 + 2y, y^2 - 2])
julia> solve(f; start_system = :total_degree)
Result with 4 solutions
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• 4 non-singular solutions (2 real)
• 0 singular solutions (0 real)
• 4 paths tracked
• random seed: 161239