$x_{n+1} = \sin(a \times y_n) + \cos(b \times x_n) - \cos(c \times z_n)$
$y_{n+1} = \sin(d \times x_n) + \cos(e \times y_n) - \cos(f \times z_n)$

$z_{n+1} = z_n + 0.1$
$z_{n+1} = x_n * y_n$
$z_{n+1} = \arctan(x_n + y_n)$
$z_{n+1} = \frac{x_n^2}{x_n + y_n}$
$z_{n+1} = (x_{n+1} - x_n)^2 + (y_{n+1} - y_n)^2$