$$x_{1} = 1 - 2^{2 y + 1}$$
=
$$1 - 2^{2 y + 1}$$
=
1 - 2^(1 + 2*y)
$$a_{1} = \frac{4 - 2 \cdot 4^{\sqrt{y}}}{2 \cdot 2^{4 y} - 5 \cdot 2^{2 y} + 2}$$
=
$$\frac{4 - 2 \cdot 4^{\sqrt{y}}}{2 \cdot 2^{4 y} - 5 \cdot 2^{2 y} + 2}$$
=
(4 - 2*4^(y^0.5))/(2 + 2*2^(4*y) - 5*2^(2*y))