
# Solving nonlinear algebraic equations

As a reader of this book you are probably well into mathematics and often "accused" of being particularly good at "solving equations" (a typical comment at family dinners!). However, is it really true that you, with pen and paper, can solve many types of equations? Restricting our attention to algebraic equations in one unknown $$x$$, you can certainly do linear equations: $$ax+b=0$$, and quadratic ones: $$ax^2 + bx + c = 0$$. You may also know that there are formulas for the roots of cubic and quartic equations too. Maybe you can do the special trigonometric equation $$\sin x + \cos x = 1$$ as well, but there it (probably) stops. Equations that are not reducible to one of the mentioned cannot be solved by general analytical techniques, which means that most algebraic equations arising in applications cannot be treated with pen and paper!

If we exchange the traditional idea of finding exact solutions to equations with the idea of rather finding approximate solutions, a whole new world of possibilities opens up. With such an approach, we can in principle solve any algebraic equation.

Let us start by introducing a common generic form for any algebraic equation: $$f(x) = 0\thinspace .$$ Here, $$f(x)$$ is some prescribed formula involving $$x$$. For example, the equation $$e^{-x}\sin x = \cos x$$ has $$f(x)= e^{-x}\sin x - \cos x\thinspace .$$ Just move all terms to the left-hand side and then the formula to the left of the equality sign is $$f(x)$$.

So, when do we really need to solve algebraic equations beyond the simplest types we can treat with pen and paper? There are two major application areas. One is when using implicit numerical methods for ordinary differential equations. These give rise to one or a system of algebraic equations. The other major application type is optimization, i.e., finding the maxima or minima of a function. These maxima and minima are normally found by solving the algebraic equation $$F'(x)=0$$ if $$F(x)$$ is the function to be optimized. Differential equations are very much used throughout science and engineering, and actually most engineering problems are optimization problems in the end, because one wants a design that maximizes performance and minimizes cost.

We restrict the attention here to one algebraic equation in one variable, with our usual emphasis on how to program the algorithms. Systems of nonlinear algebraic equations with many variables arise from implicit methods for ordinary and partial differential equations as well as in multivariate optimization. However, we consider this topic beyond the scope of the current text.

Terminology. When solving algebraic equations $$f(x)=0$$, we often say that the solution $$x$$ is a root of the equation. The solution process itself is thus often called root finding.

# Brute force methods

The representation of a mathematical function $$f(x)$$ on a computer takes two forms. One is a Python function returning the function value given the argument, while the other is a collection of points $$(x, f(x))$$ along the function curve. The latter is the representation we use for plotting, together with an assumption of linear variation between the points. This representation is also very suited for equation solving and optimization: we simply go through all points and see if the function crosses the $$x$$ axis, or for optimization, test for a local maximum or minimum point. Because there is a lot of work to examine a huge number of points, and also because the idea is extremely simple, such approaches are often referred to as brute force methods. However, we are not embarrassed of explaining the methods in detail and implementing them.

## Brute force root finding

Assume that we have a set of points along the curve of a function $$f(x)$$:

We want to solve $$f(x)=0$$, i.e., find the points $$x$$ where $$f$$ crosses the $$x$$ axis. A brute force algorithm is to run through all points on the curve and check if one point is below the $$x$$ axis and if the next point is above the $$x$$ axis, or the other way around. If this is found to be the case, we know that $$f$$ must be zero in between these two $$x$$ points.

### Numerical algorithm

More precisely, we have a set of $$n+1$$ points $$(x_i, y_i)$$, $$y_i=f(x_i)$$, $$i=0,\ldots,n$$, where $$x_0 < \ldots < x_n$$. We check if $$y_i < 0$$ and $$y_{i+1} > 0$$ (or the other way around). A compact expression for this check is to perform the test $$y_i y_{i+1} < 0$$. If so, the root of $$f(x)=0$$ is in $$[x_i, x_{i+1}]$$. Assuming a linear variation of $$f$$ between $$x_i$$ and $$x_{i+1}$$, we have the approximation $$f(x)\approx \frac{f(x_{i+1})-f(x_i)}{x_{i+1}-x_i}(x-x_i) + f(x_i) = \frac{y_{i+1}-y_i}{x_{i+1}-x_i}(x-x_i) + y_i,$$ which, when set equal to zero, gives the root $$x = x_i - \frac{x_{i+1}-x_i}{y_{i+1}-y_i}y_i\thinspace .$$

### Implementation

Given some Python implementation f(x) of our mathematical function, a straightforward implementation of the above numerical algorithm looks like

x = linspace(0, 4, 10001)
y = f(x)

root = None  # Initialization
for i in range(len(x)-1):
if y[i]*y[i+1] < 0:
root = x[i] - (x[i+1] - x[i])/(y[i+1] - y[i])*y[i]
break  # Jump out of loop

if root is None:
print 'Could not find any root in [%g, %g]' % (x[0], x[-1])
else:
print 'Find (the first) root as x=%g' % root

(See the file brute_force_root_finder_flat.py.)

Note the nice use of setting root to None: we can simply test if root is None to see if we found a root and overwrote the None value, or if we did not find any root among the tested points.

Running this program with some function, say $$f(x)=e^{-x^2}\cos(4x)$$ (which has a solution at $$x = \frac{\pi}{8}$$), gives the root 0.392701, which has an error of $$1.9\cdot 10^{-6}$$. Increasing the number of points with a factor of ten gives a root with an error of $$2.4\cdot 10^{-8}$$.

After such a quick "flat" implementation of an algorithm, we should always try to offer the algorithm as a Python function, applicable to as wide a problem domain as possible. The function should take $$f$$ and an associated interval $$[a,b]$$ as input, as well as a number of points ($$n$$), and return a list of all the roots in $$[a,b]$$. Here is our candidate for a good implementation of the brute force rooting finding algorithm:

def brute_force_root_finder(f, a, b, n):
from numpy import linspace
x = linspace(a, b, n)
y = f(x)
roots = []
for i in range(n-1):
if y[i]*y[i+1] < 0:
root = x[i] - (x[i+1] - x[i])/(y[i+1] - y[i])*y[i]
roots.append(root)
return roots

(See the file brute_force_root_finder_function.py.)

This time we use another elegant technique to indicate if roots were found or not: roots is an empty list if the root finding was unsuccessful, otherwise it contains all the roots. Application of the function to the previous example can be coded as

def demo():
from numpy import exp, cos
roots = brute_force_root_finder(
lambda x: exp(-x**2)*cos(4*x), 0, 4, 1001)
if roots:
print roots
else:
print 'Could not find any roots'

Note that if roots evaluates to True if roots is non-empty. This is a general test in Python: if X evaluates to True if X is non-empty or has a nonzero value.

## Brute force optimization

### Numerical algorithm

We realize that $$x_i$$ corresponds to a maximum point if $$y_{i-1} < y_i > y_{i+1}$$. Similarly, $$x_i$$ corresponds to a minimum if $$y_{i-1} > y_i < y_{i+1}$$. We can do this test for all "inner" points $$i=1,\ldots,n-1$$ to find all local minima and maxima. In addition, we need to add an end point, $$i=0$$ or $$i=n$$, if the corresponding $$y_i$$ is a global maximum or minimum.

### Implementation

The algorithm above can be translated to the following Python function (file brute_force_optimizer.py):

def brute_force_optimizer(f, a, b, n):
from numpy import linspace
x = linspace(a, b, n)
y = f(x)
# Let maxima and minima hold the indices corresponding
# to (local) maxima and minima points
minima = []
maxima = []
for i in range(n-1):
if y[i-1] < y[i] > y[i+1]:
maxima.append(i)
if y[i-1] > y[i] < y[i+1]:
minima.append(i)

# What about the end points?
y_max_inner = max([y[i] for i in maxima])
y_min_inner = min([y[i] for i in minima])
if y[0] > y_max_inner:
maxima.append(0)
if y[len(x)-1] > y_max_inner:
maxima.append(len(x)-1)
if y[0] < y_min_inner:
minima.append(0)
if y[len(x)-1] < y_min_inner:
minima.append(len(x)-1)

# Return x and y values
return [(x[i], y[i]) for i in minima], \
[(x[i], y[i]) for i in maxima]

The max and min functions are standard Python functions for finding the maximum and minimum element of a list or an object that one can iterate over with a for loop.

An application to $$f(x)=e^{-x^2}\cos(4x)$$ looks like

def demo():
from numpy import exp, cos
minima, maxima = brute_force_optimizer(
lambda x: exp(-x**2)*cos(4*x), 0, 4, 1001)
print 'Minima:', minima
print 'Maxima:', maxima

## Model problem for algebraic equations

We shall consider the very simple problem of finding the square root of 9, which is the positive solution of $$x^2=9$$. The nice feature of solving an equation whose solution is known beforehand is that we can easily investigate how the numerical method and the implementation perform in the search for the solution. The $$f(x)$$ function corresponding to the equation $$x^2=9$$ is $$f(x) = x^2 - 9\thinspace .$$ Our interval of interest for solutions will be $$[0,1000]$$ (the upper limit here is chosen somewhat arbitrarily).

In the following, we will present several efficient and accurate methods for solving nonlinear algebraic equations, both single equation and systems of equations. The methods all have in common that they search for approximate solutions. The methods differ, however, in the way they perform the search for solutions. The idea for the search influences the efficiency of the search and the reliability of actually finding a solution. For example, Newton's method is very fast, but not reliable, while the bisection method is the slowest, but absolutely reliable. No method is best at all problems, so we need different methods for different problems.

What is the difference between linear and nonlinear equations? You know how to solve linear equations $$ax+b=0$$: $$x=-b/a$$. All other types of equations $$f(x)=0$$, i.e., when $$f(x)$$ is not a linear function of $$x$$, are called nonlinear. A typical way of recognizing a nonlinear equation is to observe that $$x$$ is "not alone" as in $$ax$$, but involved in a product with itself, such as in $$x^3 + 2x^2 -9=0$$. We say that $$x^3$$ and $$2x^2$$ are nonlinear terms. An equation like $$\sin x + e^x\cos x=0$$ is also nonlinear although $$x$$ is not explicitly multiplied by itself, but the Taylor series of $$\sin x$$, $$e^x$$, and $$\cos x$$ all involve polynomials of $$x$$ where $$x$$ is multiplied by itself.