Abstract – Via the transposition of the theorems of Birkhoff, Chaunier-Lygeros and Lygeros-Mizony on Groups to P-Hypergroups, we explicit some relations between posets and hypergroups.

1. Definitions
is a hypergroup if : H*H→p(H) is an associative hyperoperation for which the reproduction axiom hH=Hh=H is valid for every h of H [5].

A hypergroup (H,.) is called cyclic with finite period with respect an element h of H if there exists an integer v such that H=h¹∪h²∪…∪h^v[3].

Let (G,.) be a group and P any non-empty subset of G. Then the P-hyperoperation P* is defined as follows: P*: G*G→p(G) : (x,y)→xP*y=xPy. If the reproduction axiom is valid then the hyperstructure is a P-hypergroup [6].

2. Theorems
Theorem Birkhoff [1]: If G is group then there exists a poset which automorphisms group is isomorphic to G.

Theorem Birkhoff [1]: If G is a finite group of cardinal a then there exists a poset which automorphisms group is isomorphic to G and with cardinal is equal to a²+a.

Theorem Chaunier-Lygeros [2]: Let a a prime number and n the minimal number of vertices of posets which have a automorphisms group of cardinality a. Then:
(i) n=a if a=2;
(ii) n=3a if a=3,5 or 7;
(iii) n=2a if a>7.

Posets which realize the minimum and have a minimal number of relations are respectively:
(i) ({x₀,x₁}) with x₀ and x₁ incomparable;
(ii) ({x₀,…,x_a-1,y₀,…y_a-1,z₀,…,z_a-1}) with x_i< y_i< z_i and x_i< z_j if j-i≡1 (mod a);
(iii) ({x₀,…,x_a-1,y₀,…y_a-1}) with x_i< y_j if j-i≡0,1 or 3 (mod a).

Theorem Lygeros-Mizony [5]: If G is a finite group of cardinal a, non direct product of two groups, generated by elements which are two by two of distinct order then there exists a poset which automorphisms group is isomorphic to G and with cardinal is equal to 3a.

3. New Theorems
Theorem 3.1: A poset P_o can be associated to every P-hypergroup.
Proof: By the definition of a P-hypergroup we have: G←→< G,P* > P-hypergroup and with the theorem of Birkhoff [1] we can associate a poset P_o to a group G via its automorphisms group Aut(P_o). So we have: P_o ←→ Aut(P_o) ≈ G←→ < G,P* >
Q.E.D.

Theorem 3.2: A poset P_o of cardinality a²+a can be associated to every P-hypergroup with G of cardinality a.
Proof: Corollary of the second theorem of Birkhoff.
Q.E.D.

Theorem 3.3: Let a a prime number, P-hypergroup and G a group of cardinality a=2 or 3,5,7 or 11 and more then there is an associated poset P_o of cardinality respectively a or 3a or 2a which has an automorphism group of order a.
Proof: Corollary of the theorem of Chaunier-Lygeros.
Q.E.D.

Theorem 3.4: Let P-hypergroup and G a finite group of cardinal a, non direct product of two groups, generated by elements which are two by two of distinct order then there exists a poset which automorphisms group is isomorphic to G and with cardinal is equal to 3a.
Proof: Corollary of the theorem of Lygeros-Mizony.
Q.E.D.

4. Questions
Question 1: Can we associate a general hypergroup to a poset?
Question 2: Can we associate a poset to a general hypergroup?

References

[1] Birkhoff G., 1946. Sobre los grupos de automorphimos. Revista Union Math. Arg., 11. pp.155-157.
[2] Chaunier C. and N. Lygeros 1994. Posets minimaux ayant un groupe d’automorphismes d’ordre premier. C.R.Acad.Sci. Paris, t.318, Série I, p. 695-698.
[3] De Salvo M. and Freni D. 1981. Semi-ipergruppi ciclici. Atti Sem. Mat. Fis. Un. Modena, 30, pp.44-59.
[4] Lygeros N. and Mizony M.1996. Construction de posets dont le groupe d’automorphismes est isomorphe à un groupe donné. C.R.Acad.Sci. Paris, t.322, Série I, pp. 203-206.
[5] Marty F. 1934. Sur une généralisation de la notion de groupe. 8th Congress Math. Scandinaves, Stockholm, pp.45-49.
[6] Vougiouklis Th. 1990. Isomorphisms on P-hypergroups and cyclicity. Ars Combinatoria 29A , pp.241-245.