# Proof Check: The class L is closed under concatenation

I want to prove that the class of all Turing machines that use a logarithmic amount of space is closed under concatenation. The basic idea of my proof is this: given a word, to check if it's in $L_1L_2$, simply look at all possible $n+1$ ways to slice the word into 2 parts and check if the first part belongs to $L_1$ and if the second belongs to $L_2$.

Because both of the languages can be decided in logarithmic space, checking one after the other can also be done in logarithmic space.

Is my idea correct? Thanks.

• "I want to prove that the class of all Turing machines that use a logarithmic amount of space is closed under concatenation." It's not clear to me what this means. Concatenating multiple Turing machines together to get a new one that also uses log space ? – Kurt Mueller May 3 '17 at 21:48
• Also, check-my-proof type questions aren't good form here I don't think. Usually you want to reword as a question that displays one concept, e.g. "How do I show such and such is closed under concatenation" and then in the description you can list what you've tried and what hasn't worked – Kurt Mueller May 3 '17 at 21:50

The idea is correct, but the proof is not finished. It lacks an implementation on the Turing Machine, and sometimes that can be quite tricky. Given a "slice" how are the halves "marked" as belonging to the first and second part, so we can run the two subroutines for $L_1$ and $L_2$? How are the consecutive slices generated?