On finite and countable rigid graphs and tournaments
Václav Chvátal · Czech digital mathematics library · 1965
OK FINITE AMD COUNTABLE RIGID QUFHS AND TOURNAMENTS V. CHVXTAL, Praha Let V be a non-void set and £ a binary relation on V, ECVx.V • Let f be a transformation of V .If (x,y)e.E implies (f (x) jf (y)) e £ , then f is called compatible with the relation £ • Let C(l) denote the set of all transformations compatible with a relation £ • Then C(£) with the binary operation O ( O is defined, as usual, by the compositions of transformations) is a semigroup, and its unity element is the 1dentity transformation.