Length function and characteristic sequences of quadratic algebras

In this paper we study the relations between numerical characteristics of finite dimensional algebras and such classical combinatorial objects as additive chains. We study the behavior of the length function via so-called characteristic sequences of quadratic algebras. As one of our main results, we prove the sharp upper bound for the length of quadratic algebras in terms of the Fibonacci numbers depending on the dimension of the algebra. Moreover, we show that quadratic algebras have the extremal behavior with respect to this bound. In addition, we obtain the description of the set of characteristic sequences for quadratic algebras. Namely, we completely determine the set of combinatorial properties which are satisfied for characteristic sequences of quadratic algebras and show that they belong to the family of additive chains known in combinatorics. Conversely, for a given integer sequence being an additive chain and satisfying these combinatorial properties, we construct a quadratic algebra with a characteristic sequence equal to this sequence. The obtained information on characteristic sequences is then applied to investigate the problem of realizability for the length function. In particular, we determine certain subsets of integers which are not realizable as values of the length function on quadratic algebras.