  Choosing a uniformly sampled simple directed graph realization of a degree
sequence has many applications, in particular in social networks where
self-loops are commonly not allowed. It has been shown in the past that one can
perform a Markov chain arc-switching algorithm to sample a simple directed
graph uniformly by performing two types of switches: a 2-switch and a directed
3-cycle reorientation. This paper discusses under what circumstances a directed
3-cycle reorientation is required. In particular, the class of degree sequences
where this is required is a subclass of the directed 3-cycle anchored degree
sequences. An important implication of this result is a reduced Markov chain
algorithm that uses only 2-switches.
