Two-node collocation A-stability criterion ($c_1+c_2\ge 1$)

For distinct nodes $c_1,c_2\in [0,1]$, the two-stage collocation Runge-Kutta method has stability functionThe denominator has no zeros in the closed left half-plane. On the imaginary axis the denominator's squared modulus minus the numerator's is $(d^2-r^2)y^4$. Thus $d\ge r$, equivalently $c_1+c_2\ge 1$, is necessary and sufficient for A-stability, using the maximum modulus principle and the bound at infinity.