Can. J. Math., Vol. XXVI, No. 6, 1974, pp. 1356-1371 RECURSIVELY GENERATED PERIODIC SEQUENCES R. P. KURSHAN AND B. GOPINATH 1. Introduction. A sequence (xn) (n = 1, 2, . .) is periodic if xn+p = xn for some p and all n. Periodic sequences arise naturally in geometry and arith­ metic in the study of mosaic patterns [4], continued fractions and frieze patterns [3; 5]. Some digital oscillators and tone generators also generate periodic sequences. In these cases one computes the period p of the sequence in question. On the other hand, in pseudo random sequences and cryptography [8] it is required to recursively generate sequences of large periods. We say a sequence (xn)n=i of ^-dimensional vectors is recursively generated if there exists a (vector valued) function f such that xn+i = f(xw). This is a generalization of the more usual recursion (1.1) yn = g(yn-k, . , 3>w_i) (n > k) where the y/s are scalars. The f associated with (1.1) is (1.2) f (xi, . , xk) = (x2, ...,**, g(xu . , xk)). The advantage of the representation (1.2) is that the periodicity of (yn)n can be examined through the structure of g via periodicity of f : if yn+v = yn for all n (independent of the initial values yit ... , yk), then p-io\d composition of f : fp = fofo...of satisfies P(x) = x; indeed, p f*(yi, • • •, Jic) = f -Hy2,..., y*, g(yu • • •, y*)) = ... = f(yp,yp+u • • •,yp+k-i) = (yv+u • • •,yP+k) = (yi,... ,yk). In this paper we consider those recursions whose respective generators f have a power series expansion about a fixed point; in particular, it makes sense to consider the Jacobian of f at that point (i.e., the linear part of f with respect to its power series expansion). We show that one can determine the period of f by merely computing the period of the Jacobian at zero of f, a certain affine translation of f. Using this, we provide sharp bounds for the minimum k, given the period of f, and the maximum period of f, given k, when the ground field is the rationals. The theory is applied to differential equations; and some examples are discussed. Throughout, F will denote a subfield of the complex numbers. (Actually, it suffices to assume char F = 0, if all power series are either taken to be poly­ nomials or are considered as formal series (''expanded" about 0)-thus obvi- Received May 1, 1973 and in revised form, October 26, 1973. 1356 Downloaded from https://www.cambridge.org/core. 23 Sep 2021 at 15:35:31, subject to the Cambridge Core terms of use. PERIODIC SEQUENCES 1357 ating (1.7) and concern about convergence.) Vectors and vector valued functions will be distinguished by natural bold face type, e.g., x is a vector, k k xt is the ith component of x and X = (xi, ...,xk); similarly, if U C F , f : U—» F y then f = (/i, . , fk) where ft : U —> F. The dimension k of the underlying space will be called the memory of f (cf. (1.1)), and unless otherwise stated, we assume throughout that all vector valued functions are of memory k. h Let N denote the nonnegative integers and let a = (ai, . , ak) £ N . Given a vector x £ Fk, we will use the notation x* to denote the scalar 1 ak xf • . • xk . We say that a scalar valued function g is analytic in a neighbor­ k hood U of a point x0 G F*, if there are scalars ca(a £ N ) such that for each x £ U and each 1-1 onto map co: N —> Nk the series (1.3) É *.(fl(x-Xo)-(i) 1=0 converges to g(x). In this case, instead of (1.3) we can write (1.4) g(x) = £ c.(x-Xo) without ambiguity. Note that this is equivalent to the condition that (1.3) converges absolutely to g(x), and hence the power series expansion (1.4) is unique in the sense that if XXa(x ~~ xo)a also converges absolutely to g(x) in £/, then ca = Ca for all a. We say g is analytic in an open set if g is analytic in a neighborhood of each point of that open set. We say f is analytic in an open set U if each component ft is analytic there. If f is analytic in an open set U and g is analytic in an open set containing f(Z7), then g o f is analytic in U. The proofs of this and the preceding assertions can be found in [7, §9.1-9.3]. (All functions are single-valued.) One can alternatively view f in a formal sense. Let x be a ^-dimensional indeterminate, x = (xi, . , xk) and for ca Ç F let f be the formal power series (1.5) f(x) = £ cjt. (With formal power series, operations are performed term by term, and for r f (x) = S^a'x"» f = f if and only if each cj = ca.) The term analytic will also be used to designate the formal power series (1.5); this will not lead to confusion, as assertions regarding functions analytic (in the first sense) in a neighborhood of 0 remain true when "analytic" is construed in the second (formal) sense: one must merely disregard mention of neighborhoods and domains. For f the formal power series (1.5), we define f (0) to be the "constant" term C(0 o) ; the reader should construe the phrase "f is analytic in a neighbor­ hood of a fixed point" as applied to (1.5), to mean merely: f (0) = 0. The function f will be said to have a fixed point x0 if there exists some vector x0 such that f(x0) = x0. We will use 1(1) to denote the identity function (matrix) I(x) = x(7x = x). The w-fold iteration of f : f o . of will be denoted fn. Whenever we write in we assume there is some open set U C Fk Downloaded from https://www.cambridge.org/core. 23 Sep 2021 at 15:35:31, subject to the Cambridge Core terms of use. 1358 R. P. KURSHAN AND B. GOPINATH such that f is defined on Uti V(U). Where there is a possibility of confusing fn(x) with the n-fold product/(x) -f(x) «... •/(#), the latter will be denoted A sequence of vectors (xn)n is said to be periodic of period p if xn+p = xn for all ny and p is the smallest such positive integer. If the sequence is recursively generated by f (so xn+i = f (xw) for all n), f need not be periodic in the sense that for x in the domain of f, P(x) = x (for example, fix) = —x3 recursively n generates the periodic sequence xn = ( — l) , but is not itself periodic in any open set); however, if in some set U, P(x) = x, then for any initial vector Xi Ç U, the sequence (xn)n recursively generated by f is periodic. Notice that if for some set D, f is defined in U = Utîf'CD) and P = I in D, then f (U) = U and P = I in 17. We define the period of the generator f, denoted per f, to be the smallest positive integer^ such that P = I in the domain [/of f. If (xn)n is recursively generated by a periodic f, then the period p of (xn)n divides q = per f, as xn+p = xn = P{xn) = xn+Q for all w. Notice that if f is also continuous in U, then f is a homeomorphism between U and f (£/). Furthermore, if fp is analytic in a connected open set U, and f is periodic of period p in some nonempty open subset Ur C U, then f is periodic of period p in all of U: this follows by the "principle of analytic continuation" [7, 9.4.2] which in this case says that since P = I in U', this must hold throughout U. In fact, more can be said if f is a rational function (that is, each ft is a quotient of polynomials). In this case, suppose by means of a power series expansion valid in some neighborhood U, it is determined that f is periodic of period p in U. Then each component (P)i(i = 1, . , k) is a rational function, say P JQu and in U, P i{x)/Qt(x) — xt = 0; i.e., (1.6) Pt(x) - xtQt(x) = 0 in the nonempty open set U. Hence (1.6) holds for all x Ç Fk, and thus f is periodic of period p in the complement of {x|<2(x) = 0}. On the other hand, one may think of (xn)n as a sequence of indeterminates, defined formally by xn+1 = f(xn) where f is now considered to be a formal power series. In this case, of course, the period of (xn)n and the period of f are identical. For X (E F we define per X to be the multiplicative order of X. The following proposition, of course, does not apply to formal power series. (1.7) PROPOSITION. Suppose f is periodic in a neighborhood U of a fixed point v x0. Define f'(x) = f (x + x0) — x0; then t (0) = 0 and V is analytic in a neighborhood Ur of 0. Furthermore, V is periodic in U' and the period of f in U is equal to the period of ff in U''.