Recently Alan Sokal studied the leading root $x_0(q)$ of the partial theta function $\Theta_0(x,q)=\sum\limits_{ n=0}^\infty x^nq^{ \binom n2}$, considered as a formal power series. He proved that all the coefficients of $$-x_0(q)=1+q+2q^2+4q^3+9q^4+...$$ are positive integers. I give here an explicit combinatorial interpretation of these coefficients. More precisely, I show that $-x_0(q)$ enumerates rooted trees that are enriched by certain polyominoes, weighted according to their total area.
Categories: math.CO math-ph math.MP math.NT