  Stochastic switching circuits are relay circuits that consist of stochastic
switches called pswitches. The study of stochastic switching circuits has
widespread applications in many fields of computer science, neuroscience, and
biochemistry. In this paper, we discuss several properties of stochastic
switching circuits, including robustness, expressibility, and probability
approximation.
  First, we study the robustness, namely, the effect caused by introducing an
error of size \epsilon to each pswitch in a stochastic circuit. We analyze two
constructions and prove that simple series-parallel circuits are robust to
small error perturbations, while general series-parallel circuits are not.
Specifically, the total error introduced by perturbations of size less than
\epsilon is bounded by a constant multiple of \epsilon in a simple
series-parallel circuit, independent of the size of the circuit.
  Next, we study the expressibility of stochastic switching circuits: Given an
integer q and a pswitch set S=\{\frac{1}{q},\frac{2}{q},...,\frac{q-1}{q}\},
can we synthesize any rational probability with denominator q^n (for arbitrary
n) with a simple series-parallel stochastic switching circuit? We generalize
previous results and prove that when q is a multiple of 2 or 3, the answer is
yes. We also show that when q is a prime number larger than 3, the answer is
no.
  Probability approximation is studied for a general case of an arbitrary
pswitch set S=\{s_1,s_2,...,s_{|S|}\}. In this case, we propose an algorithm
based on local optimization to approximate any desired probability. The
analysis reveals that the approximation error of a switching circuit decreases
exponentially with an increasing circuit size.
