Thoughts on Closed Position Set

Closed Position Set (CPS) is a decider that @savask reverse engineered from skelet’s code. See The closed position set decider, reverse-engineered from Skelet's program · GitHub for savask’s description.

It is a CTL-like type of decider in the sense that it builds a set of configurations described by a certain abstract language. In the case of CPS, the set it builds is described by:

A) A set of subtapes surrounding the TM head and
B) A graph/network of “continuations” for how each subtape can be extended to the left and to the right. The continuations work on blocks of symbols and say if the block at the edge of your tape is X, then here are a list of blocks {Y, Z, …} that could be added next.

A simple example is:


* Left continuations:
    0 -> {0}
    1 -> {0, 1}

* Right continuations:
    0 -> {0, 1}
    1 -> {0, 1}

* Configs:
    0 0 A> 0
    0 1 B> 0
    0 1 B> 1
    0 <C 0 1
    0 <C 1 1
    0 <D 1 0
    0 <E 1 1
    1 1 B> 0
    1 1 B> 1
    1 <C 0 1
    1 <D 1 0

In this simple case (block size 1 is not common for this proof type) the right side of the tape is unconstrained (either 0 or 1 may come after either 0 or 1) but on the left, once we see a 0, all future symbols will be 0s. Therefore, this language includes all configs: 0^inf 00 <D 10 .*, but does not contain any .* 10 <D .* which is the config required to reach halt.

After implementing this last night and thinking about it deep into the night, I woke up this morning with a realization. I think CPS is a subset of CTL! Specifically, the left and right continuations can be re-expressed as a Regular grammar and (apparently) regular grammars describe the same languages as regular expressions.

I believe that CPS cannot express all regular languages, for example: I think it cannot describe the regex (0|11)* because that would require continuations like:

00 -> {00,01,11}
01 -> {10,11}
10 -> {00,01,11}
11 -> {00,01,10,11}

But those will actually accept odd #s of 1s in a row … I don’t think it helps to increase the block size, it will have the same issue with long enough #s of 1s because I think there’s no way for CPS to have long “memory”.

Has anyone heard of this language before, does it have a name?

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I posted asking about this language on Stack Exchange Subset of regular languages - Theoretical Computer Science Stack Exchange and got a very interesting reply from a user “MRC”.

It looks like what we’re doing is kind of like an n-gram language model. We’re detecting the inherent biases of n-grams (in this case 2*block_size -grams) on the tape in order to constrain the possible configuration space enough to prove it avoids halting. I think it’s not an exact analogy because of the rigid way we break the tape into fixed blocks.

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