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# 半群理论课业代写 MT5863代写 半群理论代写 半群作业代写

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## MT5863 Semigroup theory: Problem sheet 5

Bicyclic monoid, ideals, Green’s relations

### Bicyclic monoid半群理论课业代写

The bicyclic semigroup B is defined by the presentation

b, c | bc = 1〉

and its elements are {ci bj : i, j 0}.

5-1.

Prove that an element ci bj of B is an idempotent if and only if i = j. Prove that the set E of all idempotents is a subsemigroup which is not finitely generated. 半群理论课业代写

5-2.

Consider the following two subsets of the bicyclic monoid B:

S1 = {c4i+5b4j+5 : i, j 0}

S2 = {c4i+7b4j+7 : i, j 0}.

Prove that both S1 and S2 are subsemigroups and that they are isomorphic to B. Prove that their union S = S1 ∪ S2 is also a subsemigroup. Prove that S is finitely generated.

### Ideals

5-3.

Prove that the intersection of a non-empty left ideal and a non-empty right ideal of a semigroup is always non-empty. Show by way of an example that the intersection of two left ideals may be empty. (Hint: right zero semigroup.)

5-4. Prove that a rectangular band has no proper two-sided ideals. Does it have proper left or right ideals?

### Green’s relations半群理论课业代写

5-6. Let S be the semigroup defined by the presentation

a, b | a3 = a, b4 = b, ba = a2b.

The right Cayley graph of this semigroup was determined in Problem 4-5. Use the right Cayley graph to determine the R-classes of S. Draw the left Cayley graph and determine the L -classes of S.

5-7. Consider the bicyclic monoid B = {cibj : i, j 0} (bc = 1). Prove that ciRck if and only if i = k. Prove that cibjRci . Conclude that cibjRckbl if and only if i = k. State and prove an analogous criterion for two elements of B to be L -equivalent.

5-8. Prove that an idempotent is a left identity in its R-class. State and prove an analogous assertion about L -classes.

### Further problems半群理论课业代写

5-11.* If a semigroup S is defined by a finite presentation 〈A | R with |A| > |R| then prove that S is infinite. (Hint: prove that there is a homomorphism from S onto a non-trivial subsemigroup of the additive semigroup Q.)

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