The sign of an elliptic divisibility sequence

An elliptic divisibility sequence (EDS) is a sequence of integers W_0,W_1,W_2,… generated by the nonlinear recursion satisfied by the division polyomials of an elliptic curve. We give a formula for the sign of W_n for unbounded nonsingular elliptic divisibility sequences. A typical case is Sign(W_n) = (-1)^[n*b] for an irrational real number b, where [x] denotes the greatest integer in x. As an application, we show that the associated sequence of absolute values |W_1|,|W_2|,|W_3|,… cannot be realized as the sequence counting fixed points of any (abstract) dynamical system.