# Regular Semigroups with Inverse Transversals

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1 International Mathematical Forum, Vol. 7, 2012, no. 31, Regular Semigroups with Inverse Transversals Junwei Shi Zhenji Tian School of Sciences Lanzhou University of Technology Lanzhou, Gansu, , P.R. China Abstract In this paper, a structure theorem is obtained by a permissible double (R, Λ) which is about regular semigroup with inverse transversals. It improves a structure theorem which is obtained by McAlister McFadden about regular semigroup with inverse transversals. Furthermore, we give some properties of regular semigroups with inverse transversals. Keywords: regular semigroups; inverse transversals; isomorphism 1 Introduction preliminaries In 1982, Blyth McFadden introduced regular semigroups with inverse transversals in [4], this type of semigroup has attracted much attention. An inverse subsemigroup S 0 of a regular semigroup S is an inverse transversal if V (x) S 0 = 1 for any x S, where V (x) denotes the set of inverses of x. In this case, the unique element of V (x) S 0 is denoted by x 0 (x 0 ) 0 is denoted by x 00. An inverse transversal S 0 of a regular semigroup S is a Q-inverse transversal if S 0 SS 0 S 0. Let S be an regular semigroup with an inverse transversal S 0, let R(S) ={x S x 0 x = x 0 x 00 },L(S) ={a S aa 0 = a 00 a 0 }, I(S) ={e E(S) ee 0 = e}, Λ(S) ={f E(S) f 0 f = f}, where E(S) ={x S x 2 = x} which is the idempotents of S. A b B is left [right] regular if efe = ef [efe = fe] for any e, f B. Aleft [right] inverse semigroup is an orthodox semigroup whose b of idempotents is left [right] regular. An inverse subsemigroup S 0 of a regular semigroup S is called a S- inverse transversal if I(S) Λ(S) are subsemigroups of S. In [1] Tatsuhiko

2 1548 Junwei Shi Zhenji Tian Saito first show that R(S) [L(S)] is a subsemigroup of S if only if I(S) [Λ(S)] is subsemigroup of S. In [5], Tang Xilin shows that I(S) [Λ(S)] is left [right] regular b with an inverse transversal E(S 0 ). This means that, in the terminology of [2], every inverse transversal of S is a S-inverse transversal. For convenience, R(S) is denoted by R L(S) is denoted by L, I(S) is denoted by I Λ(S) is denoted by Λ, the semilattice E(S 0 )is denoted by E 0. We list several known results which will be frequently used in this paper without special reference. Let S be an regular semigroup with an inverse transversal S 0. In [6], we know that L R = S 0,E(R) =I, E(L) =Λ,I Λ=E 0. If S is a left [right] inverse semigroup, then R = S [L = S], I= E(S) [Λ=E(S)]. If g 0 E 0, then g 0 e = g 0 e 0 for each e I fg 0 = f 0 g 0 for each f Λ. The regular subsemigroup E(S) generated by the idempotents of S is denoted by C, we denote that e = E(eCe) for any e in E(S). Clearly, ece is regular, so is e. By [6], we know that C 0 = C S 0 is an inverse transversal of C E 0 = E(C 0 ),E(C) =E(S), I C = {e C ee 0 = e} = {e E(S) ee 0 = e} = I, Λ C = {f C f 0 f = f} = {f E(S) f 0 f = f} =Λ. Let S be a regular semigroup with an inverse transversal S 0, for any a S, define λ a P S (the semigroup of partial mappings of S) as follows: λ a : aa 0 a 0 a,x a 0 xa. The composition of λ a λ b in P S is denoted by λ a λ b for any a, b S. Lemma 1.1 [6] Let S be an regular semigroup with a Q-inverse transversal S 0, then R L are orthodox semigroups. Lemma 1.2 [3] Let S be an regular semigroup with an inverse transversal S 0, e = aa 0,f = a 0 a for each a S. Then λ a is an isomorphism, its inverse is λ 1 a : f e,y aya 0. In a regular semigroup S with an inverse transversal S 0, the subsets I,Λ,R,L play important roles in studying the nature of this sort of semigroup. In this paper, a structure theorem is obtained by a permissible double (R, Λ) which is about regular semigroup with inverse transversals. It improves a structure theorem which is obtained by McAlister McFadden about regular semigroup

3 Regular semigroups with inverse transversals 1549 with inverse transversals. Furthermore, we give some properties of regular semigroups with inverse transversals. 2 The main results Theorem 2.1 Let R L be orthodox semigroups with a common Q-inverse transversal S 0. Suppose that Λ is a right regular b with an inverse transversal E 0. Let Λ R S 0 described by (e, x) e x such that, for any x, y R for any e, f Λ, (1) (e x)y = e xy f(e x) =fe x; (2) e x = ex if x E 0 or e E 0. Define a multiplication on the set R Λ={(x, e) R Λ x 0 x = e 0 } by (x, e)(y, f) =(x(e y), [(e y) 0 (e y)]f). Then R Λ is a regular semigroup with a Q-inverse transversal which is isomorphic to S 0. Conversely, every regular semigroup with a Q-inverse transversal can be constructed in this way. Proof. We give an outline of the proof. By using (1) (2), we can calculate: for any (x, e), (y, f), (z, g) R Λ, [x(e y)] 0 x(e y) =(e y) 0 (e y) ={[(e y) 0 (e y)]f} 0, [(x, e)(y, f)](z, g) =(x(e y)(f z), {[(e y)(f z)] 0 (e y)(f z)}g) =(x, e)[(y, f)(z, g)] if r, s S 0, then (r, r 0 r)(s, s 0 s)=(rs, (rs) 0 rs). Let (x, e) R Λ, then we have x(e x 0 )x = x(e x 0 x)=x(e e 0 )=x(ee 0 )=xe 0 = xx 0 x = x (e x 0 )x(e x 0 )=(e x 0 x)(e x 0 )=(ex 0 x)(e x 0 )=ee 0 (e x 0 )=e 0 (e x 0 )=e 0 e x 0 = e x 0. Thus e x 0 = x 0. By using this fact, we can prove that (x 0,x 00 x 0 ) is an inverse in the set S 0 E 0 = {(r, r 0 r) r S 0 } of (x, e). Thus R Λ is a regular semigroup containing an inverse subsemigroup S 0 E 0 which is isomorphic to S 0, each element of R Λ has an inverse in S 0 E 0. Let (r, r 0 r) be an inverse in S 0 E 0 of (x, e) R Λ. Then we have (x, e) =(x, e)(r, r 0 r)(x, e) =(x(e r)x, {[(e r)x] 0 (e r)x}e)

4 1550 Junwei Shi Zhenji Tian (r, r 0 r)=(r, r 0 r)(x, e)(r, r 0 r)=(rx(e r), [x(e r)] 0 rx(e r)). Thus we have x = x(e r)x r = rx(e r). Since e r = e rx(e r) =(e r)x(e r), thus e r = x 0. Since x 0 r 0 =(e r)r 0 = e rr 0 = e(rr 0 ), thus x 0 r 0 ia an idempotent in S 0, so Thus we have rx =(rx) 00 =(x 0 r 0 ) 0 = x 0 r 0. x 0 = e r =(e r)r 0 r = x 0 r 0 r = rxr = rxrx(e r) =rx(e r) =r. Thus S 0 E 0 is a Q-inverse transversal of R Λ. Conversely, suppose that S is a regular semigroup with a Q-inverse transversal S 0. Let Λ R S 0 be a mapping given by (e, x) e x = ex. Then the mapping satisfied (1) (2), we can constructed a regular semigroup R Λ={(x, e) R Λ x 0 x = e 0 } under a multiplication (x, e)(y, f) =(xey, (ey) 0 eyf). By defining a mapping R Λ S given by (x, e) xe, we can prove that R Λ S. Theorem 2.2 Let S be a regular semigroup with an inverse transversal S 0, then λ a 0 = λ 1 a for any a S 0. Proof. For each a S, by Lemma 1.2, we know Since λ a 0 : a 0 a 00 a 00 a 0,x a 00 xa 0. λ 1 a : a 0 a aa 0,y aya 0. R(S) ={x S x 0 x = x 0 x 00 },L(S) ={a S aa 0 = a 00 a 0 },L(S) R(S) =S 0, thus aa 0 = a 00 a 0,a 0 a = a 0 a 00 for any a S 0. It is obvious that domλ a 0 = a 0 a 00 = a 0 a = domλ 1 a, ranλ a 0 = a 00 a 0 = aa 0 = ranλ 1 a. Suppose that x a 0 a 00, then we have x = x 1 x n, where x 1,,x n E(a 0 a 00 Ca 0 a 00 ), x i = a 0 a 00 x i a 0 a 00 for any i {1,,n}, xλ a 0 = a 00 a 0 a 00 x 1 a 0 a 00 a 0 a 00 x n a 0 a 00 a 0 = aa 0 a 00 x 1 a 0 a a 0 ax n a 0 = aa 0 ax 1 a 0 a a 0 ax n a 0 aa 0 = ax 1 a 0 a a 0 ax n a 0,

5 Regular semigroups with inverse transversals 1551 then x = a 0 ax 1 a 0 a a 0 ax n a 0 a,thusx a 0 a = a 0 a 00, x i = a 0 a 00 x i a 0 a 00 = a 0 ax i a 0 a for any i {1,,n}, where x 1,,x n E(a 0 aca 0 a), xλ 1 a = aa 0 ax 1 a 0 a a 0 ax n a 0 aa 0 = ax 1 a 0 a a 0 ax n a 0, thus xλ a 0 = xλ 1 a for any a S 0. Let S be a regular semigroup with an inverse transversal S 0, let T = λ a, e = aa 0, f = a 0 a, g = bb 0, h = b 0 b for any a, b S. As the same way a S in [3], we can define a multiplication on the set T by λ a λ b = λ ab, λ ab is an isomorphism from ab(ab) 0 onto (ab) 0 ab, then we have the following result: Theorem 2.3 Let S be a regular semigroup with an inverse transversal S 0, then T is a regular semigroup with a multiplication by λ a λ b = λ ab. Proof. The operation is well-defined: For any λ a,λ b T, it is obvious that λ ab T, xλ ab =(ab) 0 x(ab). The operation is associative: (λ a λ b ) λ c = λ ab λ c = λ abc = λ a (λ b λ c ), we show that T ia a semigroup. For any λ a T, there exists λ a 0 T satisfied: λ a λ a 0 λ a = λ aa 0 λ a = λ aa 0 a = λ a. Thus we show that T is a regular semigroup. Then we have the following theorem: Theorem 2.4 Let S be a regular semigroup with an inverse transversal S 0, e = aa 0,f= a 0 a for each a S. Then (1) (eλe 0 )λ a =Λf, (f 0 If)λ 1 a = ei; (2) (ei)λ a = f 0 If,(Λf)λ 1 a = eλe 0. Proof. (1) For any m, n eλe 0, there exist s, t Λ, such that m = ese 0,n= ete 0. Then m 2 = ese 0 ese 0 = ese 0 se 0 = ese 0 = m, mn = ese 0 ete 0 = ese 0 te 0 = este 0 eλe 0. Thus eλe 0 is a b. Since eλe 0 = eλe 0 e E(eCe) e,thuseλe 0 is a subb of e. Similarly, f 0 If is a subb of f. For any x Λ, (exe 0 )λ a =(aa 0 xa 00 a 0 )λ a = a 0 xa 00 a 0 a = a 0 xa 00 f,

6 1552 Junwei Shi Zhenji Tian (a 0 xa 00 ) 0 a 0 xa 00 = a 0 x 0 a 00 a 0 xa 00 = a 0 x 0 xa 00 = a 0 xa 00 Λ, then we have (exe 0 )λ a Λf, thus(eλe 0 )λ a Λf. Conversely, for any y Λ, we know that yf = ya 0 a = a 0 a 00 ya 0 a = a 0 ea 00 ya 0 e 0 a, (a 00 ya 0 ) 0 a 00 ya 0 = a 00 y 0 a 0 a 00 ya 0 = a 00 y 0 ya 0 = a 00 ya 0 Λ, then we have yf =(ea 00 ya 0 e 0 )λ a (eλe 0 )λ a,thusλf (eλe 0 )λ a. We obtain that (eλe 0 )λ a =Λf. Similarly, we can show that (f 0 If)λ 1 ei. (2) For any g I, we have (eg)λ a = a 0 ega = a 0 ga = a 0 a 00 a 0 ga = a 0 a 00 a 0 ga 00 a 0 a = f 0 a 0 ga 00 f, a 0 ga 00 (a 0 ga 00 ) 0 = a 0 ga 00 a 0 g 0 a 00 = a 0 gg 0 a 00 = a 0 ga 00 I. Thus we obtain that (ei)λ a f 0 If. Conversely, for any h I, we have f 0 hf = a 0 a 00 ha 0 a = a 0 ea 00 ha 0 a, a 00 ha 0 (a 00 ha 0 ) 0 = a 00 ha 0 a 00 h 0 a 0 = a 00 hh 0 a 0 = a 00 ha 0 I, so f 0 hf =(ea 00 ha 0 )λ a (ei)λ a, we obtain that f 0 If (ei)λ a. Thus (ei)λ a = f 0 If. Similarly, we can show that (Λf)λ 1 a = eλe 0. a = References [1] Saito T. A Note on Regular Semigroups with Inverse Transversals. Semigroup Forum, Vol. 33 (1986): [2] Saito T. Construction of Regular Semigroups with Inverse Transversals. Proc. Edinburgh Math. Soc., 32 (1989): [3] Guangtian Song Fenglin Zhu. Fundamental Regular Semigroups with Inverse Transversals. Semigroup Forum, Vol. 67 (2003): [4] Blyth T. S. McFadden R. B. Regular Semigroups with a Multiplicative Inverse transversal. Proc. Edinburgh Math. Soc., 92A (1982): [5] Xi Lin Tang. Regular Semigroups with Inverse Transversals. Semigroup Forum, Vol. 55 (1997), [6] Nan Cai. Regular Semigroups with Inverse Transversals. Science Technology Engineering. Vol. 67 (2007): Received: January, 2012

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