We describe all serial posets with positive-definite quadratic Tits form and prove that any poset of order greater than 7 with positive-definite Tits form is serial.Quadratic forms are encountered in the solution of various problems in algebra, geometry, the theory of differential and integral equations, operator theory, and other fields of mathematics (see, e.g., ). Among them, an important role is played by quadratic Tits forms for oriented graphs, posets, algebras, etc. In the present paper, we consider exactly these forms.
Formulation of the Main ResultFirst, recall some definitions. Let S be a finite or an infinite poset. We say that S is the sum of its subsets A 1 , . . . , A s and writeIf all elements of different terms are always incomparable, then S is called the direct sum of the indicated subsets. Further, according to [27], the sum S = A 1 + . . . + A s is called one-sided if (up to enumeration of terms) one has i < j whenever there exist elements b ∈ A i and c ∈ A j for i = j such that b < c. According to [27], the sum S = A 1 + . . . + A s is called minimax if it follows from the relation x < y, where x and y belong to different terms, that x and y are, respectively, minimal and maximal elements of the set S. Formally, a direct sum is minimax. Nevertheless, considering minimax sums in what follows, we always assume for convenience that they are not direct.A subset of a poset S is understood as a complete partially ordered subset, i.e., a partial order on it is induced by a partial order on S.The form q S (z) : Z S∪0 0 → Z defined by the equality