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For example, if is defined on the Real Number s by : then 2 is a fixed point of , because . Not all functions have fixed points: for example, if is a function defined on the real numbers as , then it has no fixed points, since is never equal to for any real number. In graphical terms, a fixed point means the point is on the line , or in other words the Graph of has a point in common with that line. The example is a case where the graph and the line are a pair of Parallel lines. Points which come back to the same value after a finite number of Iterations of the function are known as Periodic Point s; a fixed point is a periodic point with period equal to one. ATTRACTIVE FIXED POINTS ''x''''n''+1 = cos ''x''''n'' with initial value ''x''1 = -1.]] An attractive fixed point of a function ''f'' is a fixed point ''x''0 of ''f'' such that for any value of ''x'' in the domain that is close enough to ''x''0, the Iterated Function sequence : Converge s to ''x''0. How close is "close enough" is sometimes a subtle question. The natural Cosine function ("natural" means in Radians , not degrees or other units) has exactly one fixed point, which is attractive. In this case, "close enough" is not a stringent criterion at all -- to demonstrate this, start with ''any'' real number and repeatedly press the ''cos'' key on a calculator. It quickly converges to about 0.73908513, which is a fixed point. That is where the graph of the cosine function intersects the line . Not all fixed points are attractive: for example, is a fixed point of the function , but iteration of this function for any value other than zero rapidly diverges. Attractive fixed points are a special case of a wider mathematical concept of Attractor s. An attractive fixed point is said to be a stable fixed point if it is also Lyapunov Stable . A fixed point is said to be a neutrally stable fixed point if it is Lyapunov Stable but not attracting. The center of a Second Order Homogeneous Linear Differential Equation is an example of a neutrally stable fixed point. THEOREMS GUARANTEEING FIXED POINTS There are numerous theorems in different parts of mathematics that guarantee that functions, under certain circumstances, must have one or more fixed points. These are amongst the most basic qualitative results available: such Fixed-point Theorem s that apply in generality are very valuable insights. APPLICATIONS In many fields, equilibria or Stability are fundamental concepts that can be described in terms of fixed points. For example, in Economics , a Nash Equilibrium of a Game is a fixed point of the game's Best Response Correspondence . In Compilers , fixed point computations are used for whole program analysis, which are often required to do code Optimization . The vector of PageRank values of all web pages is the fixed point of a Linear Transformation derived from the World Wide Web 's link structure. Logician Saul Kripke makes use of fixed points in his influential theory of truth. He shows how one can generate a partially defined truth predicate (one which remains undefined for problematic sentences like "This sentence is not true"), by recursively defining "truth" starting from the segment of a language which contains no occurrences of the word, and continuing until the process ceases to yield any newly well-defined sentences. (This will take a denumerable infinity of steps.) That is, for a language L, let L-prime be the language generated by adding to L, for each sentence S in L, the sentence "''S'' is true." A fixed point is reached when L-prime is L; at this point sentences like "This sentence is not true" remain undefined, so, according to Kripke, the theory is suitable for a natural language which contains its ''own'' truth predicate. SEE ALSO
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