# Segments

# Segments#

All phases in Dymos are decomposed into one or more *segments* in time. These segments
serve the following purposes:

Gauss-Lobatto collocation and the Radau Pseudospectral method model each state variable as a polynomial segment in nondimensional time within each segment.

Each control is modeled as a polynomial in nondimensional time within each segment.

The order of the *state* polynomial segment is given by the phase argument `transcription_order`

.
In Dymos the minimum supported transcription order is 3.

State-time histories within a segment are modelled as a Lagrange polynomial.
Continuity in state value may be enforced via linear constraints at the segment boundaries (the default behavior) or by specifying a *compressed* transcription whereby the state value at a segment boundary is provided as a single value.
The default *compressed* transcription yields an optimization problem with fewer variables, but in some situations using uncompressed transcription can result in more robust convergence.