To verify safety and robustness of neural networks, researchers have successfully applied
abstract interpretation
, primarily using the interval abstract domain. In this paper, we study the theoretical power and limits of the interval domain for neural-network verification.
First, we introduce the
interval universal approximation
(IUA) theorem. IUA shows that neural networks not only can approximate any continuous function
f
(universal approximation) as we have known for decades, but we can find a neural network, using any well-behaved activation function, whose interval bounds are an arbitrarily close approximation of the set semantics of
f
(the result of applying
f
to a set of inputs). We call this notion of approximation
interval approximation
. Our theorem generalizes the recent result of Baader et al. from ReLUs to a rich class of activation functions that we call
squashable functions
. Additionally, the IUA theorem implies that we can always construct provably robust neural networks under ℓ
∞
-norm using almost any practical activation function.
Second, we study the computational complexity of constructing neural networks that are amenable to precise interval analysis. This is a crucial question, as our constructive proof of IUA is exponential in the size of the approximation domain. We boil this question down to the problem of approximating the range of a neural network with squashable activation functions. We show that the range approximation problem (RA) is a Δ
2
-intermediate problem, which is strictly harder than
NP
-complete problems, assuming
coNP
⊄
NP
. As a result,
IUA is an inherently hard problem
: No matter what abstract domain or computational tools we consider to achieve interval approximation, there is no efficient construction of such a universal approximator. This implies that it is hard to construct a provably robust network, even if we have a robust network to start with.