Fixed Points and The Fixed Point Algorithm

Fixed Points A fixed point for a function f(x) is a value x 0 in the domain of the function such that f(x 0 ) = x 0. We say the function f(x) fixes the value x 0. Geometry Geometrically the fixed point occurs where the graph of y=f(x) crosses the graph of y=x. A function may have none, one or many fixed points. Algebra In terms of algebra the fixed point(s) is(are) the solutions to the equation f(x)=x. In the example to the right we see the fixed point for the function f(x) is 2. If you compute f(2) you get 2 (i.e. f(2)=2 ). x y y=x 2

Complicated Fixed Points Finding the fixed point for some functions results in a very complicated or impossible equation to solve that would find and exact value for the fixed point. For example if we consider the function f(x)= cos( x ) it is apparent from the graph that (or you could prove using the Intermediate Value Theorem) this functions has a fixed point. It has been proven there is no algebraic combination of number to express the solution to the equation cos( x )= x. This is why we need to rely on Numerical Method to estimate solutions. The Fixed Point Algorithm The Fixed Point Algorithm (FPA) is an algorithm that generates a recursively defined sequence that will find the fixed point for a function under the correct conditions. One of the big advantages of the algorithm is that it is no very difficult to implement.

The Fixed Point Algorithm (FPA) use a value x 0 (ideally chosen close to the fixed point you want to find) and a function f(x) and generates a recursively defined sequence given by: x 0 for n=0 and x n+1 =f(x n ) for n>0. The FPA will be able to estimate a fixed point if and only if the sequence x n converges. There are several conditions that will that would imply convergence. f(x) is increasing and bounded f(x) satisfies a Lipshitz condition f(x) is decreasing and contractive others