Question 43
Let $a, b, c, d$ be strictly positive reals such as $a+b+c+d=1$. Show that: $$
\frac{a^{4}}{a^{3}+a^{2} b+a b^{2}+b^{3}}+\frac{b^{4}}{b^{3}+b^{2} c+b c^{2}+c^{3}}+\frac{c^{4}}{c^{3}+c^{2} d+c d^{2}+d^{3}}+\frac{d^{4}}{d^{3}+d^{2} a+d a^{2}+a^{3}} \geqslant \frac{1}{4}
$$ and determine equality cases.
\frac{a^{4}}{a^{3}+a^{2} b+a b^{2}+b^{3}}+\frac{b^{4}}{b^{3}+b^{2} c+b c^{2}+c^{3}}+\frac{c^{4}}{c^{3}+c^{2} d+c d^{2}+d^{3}}+\frac{d^{4}}{d^{3}+d^{2} a+d a^{2}+a^{3}} \geqslant \frac{1}{4}
$$ and determine equality cases.