§8.6: Highlight: Randomized decision tree complexity

A decision tree $T$ for $f : \{-1,1\}^n \to \{-1,1\}$ can be thought of as a deterministic algorithm which, given adaptive query access to the bits of an unknown string $x \in \{-1,1\}^n$, outputs $f(x)$. E.g., to describe a natural decision tree for $f = \mathrm{Maj}_3$ in words: “Query $x_1$, then $x_2$. If they [...]

§3.2: Subspaces and decision trees

In this section we treat the domain of a boolean function as ${\mathbb F}_2^n$, an $n$-dimensional vector space over the field ${\mathbb F}_2$. As mentioned earlier, it can be natural to index the Fourier characters $\chi_S : {\mathbb F}_2^n \to \{-1,1\}$ not by subsets $S \subseteq [n]$ but by their $0$-$1$ indicator vectors $\gamma [...]