2. Determine all singular points, if any, and classify each insofar as possible.
(a) $x' = y$, $y' = 1 - x^4$
(b) $x' = 1 - y^2$, $y' = 1 - x$
(c) $x' = y$, $y' = \frac{(1 - x^2)}{(1 + x^2)}$
(d) $x' = x - y$, $y' = \sin(x + y)$
(e) $x' = (1 - x^2)y$, $y' = -x - 2y$
(f) $x' = (1 - x^2)y$, $y' = -x + 2y$
(g) $x' = -2x - y$, $y' = x + x^3$
(h) $x' = -2x - y$, $y' = \sin x$
(i) $x' = ye^x - 1$, $y' = y - x - 1$
(j) $x' = x^2 - 2y$, $y' = 2x - y$
(k) $x' = x^2 - y^2$, $y' = x^2 + y - 2$
(l) $x' = y$, $y' = -3\sin x$
(m) $x' = x + 2y$, $y' = -x - \sin y$
(n) $x' = (x^6 + 1)y$, $y' = x^2 - 4$
