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LNCS 2950 - Aspects of Molecular Computing (Frontmatter Pages)

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Here the last one holds if and only if<br />

Fixed Point Approach to Commutation <strong>of</strong> Languages 125<br />

From (4) and (6) we obtain the equivalence<br />

(bab) k−1 ab(bab) n+1 ∈ Xi. (6)<br />

(bab) k ab(bab) n ∈ Xi+1 ⇐⇒ (bab) k+1 ab(bab) n−1 , (bab) k−1 ab(bab) n+1 ∈ Xi (7)<br />

Now, the result follows by induction, when we cover the cases k =0orn =0.<br />

In the case k =0andn ≥ 0wehavethatab(bab) n ∈ X0, but ab(bab) n /∈ X1,<br />

since a · ab(bab) n /∈ X0X. The same applies also for (bab) n ba by symmetry.<br />

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Fig. 1. Language (bab) ∗ ab(bab) ∗ written as a pyramid.<br />

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� � � � � � �<br />

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Fig. 2. Deleting words in (bab) ∗ ab(bab) ∗ from languages Xi during the iteration.<br />

In the case n =0andk ≥ 1wefirstnotethatX(bab) k ab ⊆ X0X, due to the<br />

equations in (3) and the fact that bab·(bab) k ab =(bab) k ba·bab ∈ X0X. Similarly,<br />

(bab) k abX ⊆ XX0, due to the equations in (5) and the fact (bab) k ab · bab =<br />

bab · (bab) k−1 ab(bab) ∈ XX0. These together imply that (bab) k ab ∈ X1. On<br />

the other hand, since (bab) k ba /∈ X1, thenbab · (bab) k ab /∈ X1X, and hence<br />

(bab) k ab /∈ X2.

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