Consider the following one-parameter systems of differential equations:

(a) ˙ x = x, ˙ y = μ − y4;

(b) ˙ x = x2 − xμ2, ˙ y = −y;

(c) ˙ x = −x4 + 5μx2 − 4μ2, ˙ y = −y.

Find the critical points, plot phase portraits, and sketch a bifurcation diagram in each case.