13 Let $a \geqslant b \geqslant c \geqslant d>0$, try to find the minimum value of the function $f(a, b, c, d)=$ $\left(1+\frac{c}{a+b}\right)\left(1+\frac{d}{b+c}\right)\left(1+\frac{a}{c+d}\right)\left(1+\frac{b}{d+a}\right)$.