# Solve Examples

## A First Example (no start system)

We can solve the system $F(x,y) = (x^2+y^2+1, 2x+3y-1)$ in the following way

@var x y
F = System([x^2+y^2+1, 2x+3y-1], variables = [x, y])
solve(F)
Result with 2 solutions
=======================
• 2 paths tracked
• 2 non-singular solutions (0 real)
• random_seed: 0x1c827752
• start_system: :polyhedral


Here, the call

F = System([x^2+y^2+1, 2x+3y-1], variables = [x, y])

also determines the ordering of the variables in the solution vectors. By default, variables are ordered lexciographically. If this is okay, you can also call solve without first constructing a system, i.e.,

solve([x^2+y^2+1, 2x+3y-1])

## Parameter Homotopy

Using the syntax

solve(F, startsolutions; start_parameters, target_parameters)

We can track the given start solutions alogn the parameter homotopy

$H(x, t) = F(x, tp₁+(1-t)p₀),$

where $p₁$ (=start_parameters) and $p₀$ (=target_parameters) are vectors of parameter values for $F$ where $F$ is a System depending on parameters.

Assume we want to perform a parameter homotopy $H(x,t) := F(x; t[1, 0]+(1-t)[2, 4])$ where

$F(x; a) := (x₁^2-a₁, x₁x₂-a₁+a₂)$

and let's say we are only interested in tracking the solution $[1,1]$. This can be accomplished as follows

@var x[1:2] a[1:2]
F = System([x[1]^2-a[1], x[1]*x[2]-a[1]+a[2]], parameters = a)
start_solutions = [[1, 1]]
p₁ = [1, 0]
p₀ = [2, 4]
solve(F, start_solutions; start_parameters=p₁, target_parameters=p₀)
Result with 1 solution
======================
• 1 path tracked
• 1 non-singular solution (1 real)
• random_seed: 0x19cb8e6d


## Start Target Homotopy

solve(G, F, start_solutions; options...)

This constructs the homotopy $H(x,t) = tG(x)+(1-t)F(x)$ to compute solutions of the system F. start_solutions is a list of solutions of G which are tracked to solutions of F.

@var x y
G = System([x^2+1,y+1])
F = System([x^2+y^2+1, 2x+3y-1])
solve(G, F, [[im, -1], [-im, -1]])
Result with 2 solutions
=======================
• 2 paths tracked
• 2 non-singular solutions (0 real)
• random_seed: 0xf5e83f70