  Classes of target functions containing a large number of approximately
orthogonal elements are known to be hard to learn by the Statistical Query
algorithms. Recently this classical fact re-emerged in a theory of
gradient-based optimization of neural networks. In the novel framework, the
hardness of a class is usually quantified by the variance of the gradient with
respect to a random choice of a target function.
  A set of functions of the form $x\to ax \bmod p$, where $a$ is taken from
${\mathbb Z}_p$, has attracted some attention from deep learning theorists and
cryptographers recently. This class can be understood as a subset of
$p$-periodic functions on ${\mathbb Z}$ and is tightly connected with a class
of high-frequency periodic functions on the real line.
  We present a mathematical analysis of limitations and challenges associated
with using gradient-based learning techniques to train a high-frequency
periodic function or modular multiplication from examples. We highlight that
the variance of the gradient is negligibly small in both cases when either a
frequency or the prime base $p$ is large. This in turn prevents such a learning
algorithm from being successful.
