Chapter 17 - CLosure under +, . and *

just got a question on this,

Its seems pretty easy to show that a language(s) L1 and L2 are closed under +, product and * (kleene closure),

but in our 2006 exam paprer they ask

1. Show the language L1L2 and
2. prove that it is correct (two way proof).

I need to know what they want from number 2. The number 1 is the proof in Cohen.

Danie van Eeden
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Danie, I think you'll find the phrase "two way proof" either in your study guide or in the exam tuts. Sorry I can't be more helpful than that right now. (I have my own dragons to slay).

It's this kind of proof from assertion A to assertion B ... A -> B (fill in an A and

Followed by B -> A.

So in the end you should have A <-> B.

(The basics are old stuff, in other words.)

And memory deserts me there for now.

i think the lecturer added something like on the discussion forum...