The exclusion of knots can be formulated in the following way: if
Both of the structures above constitute a knot.
Knots, however, do occur in a few RNA molecules. Thus, this constraint might seem somewhat arbitrary. However, knots also tend to add needless mathematical complexity, and little is known about the thermodynamics of knots. Thus knots are often regarded as part of the tertiary structure of RNA and the problem of detecting them is not considered when determining the secondary structure.