1. Computability Kolmogorov-Chaitin-Solomonoff. Other topics. Homework: Prepare presentations.

2. Information • Shannon definition: series of (binary) choices. Information in measured in bits. • Computability definition: Let x be a binary string. A minimal description of x, d(x) is the shortest string,<M,w>, where Turing Machine M on input w halts with x on the tape. The descriptive complexity (aka Kolmogorov or Kolmogorov-Chaitin complexity) is K(x) = |d(x)| • the length of this shortest string. • Note: there may be more than one.

3. Informally • Suppose we have a string consisting of 100 groups of '0110'. This is a description and it seems like it would be shorter than writing out the whole string. • The formal description includes the TM (or program) that knows what to do with 100 groups of something. The [full] minimal description consists of this TM plus an encoding of 100 groups, 0110. • Program that takes description and produces string. • This definition requires a TM for even the definition that is the whole string.

4. Common example • Mandelbrot fractals are very intricate patterns and yet can be produced (re-produced?) by simple computer programs.

5. Claim • There exists a constant c, such that for all x, K(x)<= |x| + c. • intuitively: take a fixed TM that halts immediately. Then it halts with the string x on it. • There exists a constant c, such that for all x and y, K(xy)<=2K(x) + K(y)+c

6. Incompressible strings • Definitions: a string x is c-compressible if K(x)<=|x|-c. • If x is not c-compressible, x is incompressible by c. • If x is incompressible by 1, x is incompressible.

7. Incompressible strings exist! • The number of strings of length n is greater than the number of descriptions of length less than n. So, some string of length n is not described by any description of length less than n.

8. K(x) is not computable! • [and not because the definition is tied to any one of several models of computing] • See http://en.wikipedia.org/wiki/Kolmogorov_complexity It is a Halting Problem type of proof.

9. Berry Paradox • Let n be the smallest positive integer that cannot be defined in fewer than twenty English words. • oops! • Relates also to Godel incompleteness results.

10. Recursion Theorem also called fixed point theorem. • There exists a program that prints itself. • General strategy: create constant that represents working part of program and write program that prints out constant. • See Logo example in Shai lectures. • c# example next slide:http://igoro.com/archive/how-to-write-a-self-printing-program/

11. c# example May need to force to be in two lines class P{static void Main(){var S="class P{{static void Main(){{var S={1}{0}{1};System.Console .Write(S,S,'{1}');}}}}";System.Console.Write(S,S,'"');}}

12. finish Breaking the code • Discussion

13. Homework • Prepare presentations!