# Self replicating software – Part 2 – Recursion theorem proof

In this post I’ll cover the proof of the Recusion theorem (see Self Replicating Software – Part 1 – The Recursion Theorem).

The proof for the Recursion theorem is a constructive proof, meaning that a Turing Machine (TM) that can reference its own description is constructed. This proof was taken from Michael Sipser’s “An Introduction to the Theory of Computation”.

Before we get into the proof of the Recursion Theorem, we’ll need a special TM. There is a computable function q which can print out the description of a TM that prints the input that was given to q.

So, if we have a machine P_w, that prints out w (where w is input to the machine), q(w) prints out the description of TM_w. The construction of such a machine is relatively simple. Lets call the machine that computes q(w) Q_w. Here is an informal description:

```Q_w = " Step 1: Create a machine P_w =" Step 1.1: print w"```

Step 2: Print <P_w>

Recall that <P_w> is notation for “The description of the Turing Machine P_w”.

Last time, we had a TM called TM_T that took two inputs, one being the description of a TM, the other input called w, and performed some computation with it. We also had another TM called TM_R which took one input, namely the same w passed to TM_T and performed the same computation as TM_T (on the description of a TM) but with it’s own description (<TM_R>) filled in. Since TM_R only takes one input, w, the description of TM_R is internal to TM_R.

The TM_R is constructed in a special manner, namely it has 3 components. Let’s call these TM_A, TM_B, and TM_T where TM_T is the same TM_T referenced before. TM_A is a special TM that generates a description of TM_B and TM_T. TM_B computes the description of TM_A. The tricky part is to avoid creating a cyclic reference. In essence, you can’t have TM_A print out <TM_B><TM_T> and then have TM_B print out <TM_A> since they reference each other directly. The trick is to have TM_B calculate <TM_A> by using the output from TM_A.

Think about this abstractly for a moment, if TM_A were to print some string w, then <TM_A> is simply q(w) (a.k.a. the description of a machine that prints w). In case of the recursion theorem, TM_A is the machine that prints out <TM_B><TM_T> and TM_B prints the description of a machine that prints out <TM_B><TM_T>

So, the proof goes as follows:

```TM_R(w) = " TM_A which is P_<TM_B><TM_T> TM_B which is Q_w TM_T which is specified ahead of time```

```Step 1: Run TM_A Step 2: Run TM_B on the output of TM_A Step 3: Combine the output of TM_A with the output of TM_B and w Step 4: Pass control to TM_T "```

Think about this, TM_A is a machine that prints out <TM_B><TM_T>. <TM_B> computes the description of a machine that prints out <TM_B><TM_T>. Note the following:

```TM_A = P_<TM_B><TM_T> <TM_A> = <P_<TM_B><TM_T>> <P_<TM_B><TM_T>> = Q(<TM_B><TM_T>)```

TM_B = Q(w) where w is <TM_B><TM_T>

If TM_T prints the description, then TM_T is called a quine. For TM_T to be a virus, it would “print” the description to a file (somewhere on the tape).

We’ve covered a fair amount of theory between this post and the last post so let’s put it into some concrete code. In this example, TM_T will just print the description of the machine. This type of program is known as a quine or selfrep.

```[lost@pagan code]\$ grep -n ".*" quine.py 1:#!/usr/bin/python 2: 3:def TM_T(M): 4: print M; 5: 6:def TM_B(w): 7: computedA = '# Below is TM_A' + chr(0xA) + 's = ' + '"' + '""' + w + '"' + '"";' + chr(0xA) + 'TM_B(s);'; 8: TM_T(w + computedA); 9: 10:# Below is TM_A 11:s = """#!/usr/bin/python 12: 13:def TM_T(M): 14: print M; 15: 16:def TM_B(w): 17: computedA = '# Below is TM_A' + chr(0xA) + 's = ' + '"' + '""' + w + '"' + '"";' + chr(0xA) + 'TM_B(s);'; 18: TM_T(w + computedA); 19: 20:"""; 21:TM_B(s); [lost@pagan code]\$ clear [lost@pagan code]\$ grep -n ".*" quine.py 1:#!/usr/bin/python 2: 3:def TM_T(M): 4: print M; 5: 6:def TM_B(w): 7: computedA = '# Below is TM_A' + chr(0xA) + 's = ' + '"' + '""' + w + '"' + '"";' + chr(0xA) + 'TM_B(s);'; 8: TM_T(w + computedA); 9: 10:# Below is TM_A 11:s = """#!/usr/bin/python 12: 13:def TM_T(M): 14: print M; 15: 16:def TM_B(w): 17: computedA = '# Below is TM_A' + chr(0xA) + 's = ' + '"' + '""' + w + '"' + '"";' + chr(0xA) + 'TM_B(s);'; 18: TM_T(w + computedA); 19: 20:"""; 21:TM_B(s);```

[lost@pagan code]\$ ./quine.py | md5sum; md5sum quine.py
dd9958745db4735ceab8101cae0a15a8 –
dd9958745db4735ceab8101cae0a15a8 quine.py
(Note the md5sum of the output and the md5sum of the input are the same)

Let’s analyze this piece by piece. In this program, TM_A is represented by lines 11 – 21. TM_A is the description of TM_B and TM_T. Notice that TM_A is a copy of (the descriptions of) TM_B and TM_T and stuck in triple quotes (one method in Python for representing strings).

TM_B is on lines 6-10 (including the comment “# Below is TM_A”). Recall that TM_B computes the description of TM_A using the output from TM_A. The input to TM_B is the output of TM_A, which was the description of TM_B and TM_T. Finally, the output from TM_B is transferred to TM_T which prints the contents.

Here is the logic flow:

```Lines 11-20 calculate P_<TM_B><TM_T> Line 21 takes the output (s) and passes it to TM_B Line 7 computes the description of TM_A (computedA) from the output of TM_A (which was passed to TM_B) Line 8 combines the computed description of TM_A with the output of TM_A and passes the whole string to TM_T. Line 4 prints the description of the machine```

In the case of a virus, instead of printing the description to the screen, a virus would print the description to another python file.

What we’ve done is prove (from a theoretical point of view) why viruses can exist. The fact viruses exist already tells us this, but we’ve now proven from an abstract view that they do exist. We’ve also given a basic algorithm for creating viruses, and shown how to implement something simple in Python.

In the next post we’ll cover another method (perhaps more commonly used) a virus can use to get it’s own description (which doesn’t depend on the recursion theorem).