= Tricritical wing coexistence factorization
{title2=$f-f(a)=\frac v6(M-a)^2(M-b)^2[(M+s)^2+p]$}
Take two nonnegative coexisting minima $a\leq b$, put $s=a+b$ and $p=ab$, and fix $v>0$. For the sextic <Landau free energy> with quadratic, quartic, sextic and linear terms, equal stationary minimum values occur at
$$
u=\frac{2v}{3}(-2s^2+3p),\qquad
r=\frac v3(s^4-s^2p+3p^2),\qquad h=\frac v3s^3p.
$$
At these coefficients,
$$
f(M)-f(a)=\frac v6(M-a)^2(M-b)^2[(M+s)^2+p]\geq0.
$$
Thus both are <global minima>. The case $a=0$ gives the zero-field three-phase line, including the negative minimum; the limit $a=b$ gives the <tricritical wing critical edge>. Reflecting $M,h$ gives the negative-field wing.
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