It is proved that the Jacobson radical of a monomial algebra over a field is nil and a description of elements of the Jacobson radical is obtained.In what follows || £>|| will denote the cardinality of a set D, Z the ring of integers, F a field, Z = {zi, Z2, • • •, z n ) a finite set, (Z) the free semigroup on Z with unit element, and F{Z) the free F-algebra on Z. Given any subset Y c (Z), the factor algebra A = F(Z)/(Y) is called the monomial algebra defined by the set Y of words of (Z), where (7) is the ideal of F{Z) generated by Y. Note that(1)We denote by Nil (A) and J (A) the nil-radical and the Jacobson radical of A respectively. Next, we set X = {*, = z, + (Y) \ i = 1, 2,..., n}. Let,Xi 2 , ..., Xi m € X. The product u = Xi t Xj 2 ... Xj m e A is called a word in A, and we set m, if u ^ 0; |u| = 0, if w = 1 (i.e., m =0); [-1, if H = 0.In view of (1) the number |u| is well-defined. The number |«| is called the length of u. Let 1 < k < I < m and w = • • We shall call w a subword ofu and denote this fact by w An infinite sequence W = {Wj | w e W, j e Z} is said to be an infinite word in A. A subword u; of a product of the form WjWj + \... wj + / c is called a finite subword of W (and this fact is denoted by w W). An infinite word W is said to be nonzero if all of its finite subwords are nonzero.
Definition 1.A nonzero infinite word W is called uniformly recurrent if for any integer k there exists an integer L = L(k) such that for any two finite subwords u and v of W with |u| = k and |u| = L we have thatThe following theorem is the main result of the present paper. Theorem 1. Let A be a finitely generated monomial algebra. Then J (A) = Nil(A). Furthermore, as a linear space, J (A) is spanned by all words which are not subwords of any uniformly recurrent word in A. 'Partially supported by the Russian Foundation for Fundamental Research.