We show a deterministic simulation (or lifting) theorem for composed problems $f \circ \EQ_n$ where the inner function (the gadget) is Equality on $n$ bits. When $f$ is a total function on $p$ bits, it is easy to show via a rank argument that the communication complexity of $f\circ \EQ_n$ is $\Omega(\deg(f) \cdot n)$. However, there is a surprising counter-example of a partial function $f$ on $p$ bits, such that any completion $f'$ of $f$ has $\deg(f') = \Omega(p)$, and yet $f \circ \EQ_n$ has communication complexity $O(n)$. Nonetheless, we are able to show that the communication complexity of $f \circ \EQ_n$ is at least $D(f) \cdot n$ for a complexity measure $D(f)$ which is closely related to the $\AND$-query complexity of $f$ and is lower-bounded by the logarithm of the leaf complexity of $f$. As a corollary, we also obtain lifting theorems for the set-disjointness gadget, and a lifting theorem in the context of parity decision-trees, for the $\NOR$ gadget.

In this talk, I will talk about simulations or lifting theorems in general from a previous work of Chattopadhyay et al., and lifting theorem for Equality in particular—especially why the general recipe of Chattopadhyay et al. does not work for Equality. I will also mention an application of this technique to prove tight communication lower bound for Gap-Equality problem. This is a joint work with Bruno Loff.