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Ustop doc #77

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844ad74
Updating docs on storage and ustop
Jun 9, 2021
b895dc8
one more doc update
Jun 9, 2021
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5 changes: 3 additions & 2 deletions braid/braid.h
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Original file line number Diff line number Diff line change
Expand Up @@ -121,8 +121,9 @@ typedef struct _braid_Vector_struct *braid_Vector;
* The user must advance the vector *u* from time *tstart* to *tstop*. The time
* step is taken assuming the right-hand-side vector *fstop* at time *tstop*.
* The vector *ustop* may be the same vector as *u* (in the case where not all
* unknowns are stored). The vector *fstop* is set to NULL to indicate a zero
* right-hand-side.
* unknowns are stored, for more details on *ustop*, see
* [Some Advanced Features](@ref advancedfeatures) in the User Manual).
* The vector *fstop* is set to NULL to indicate a zero right-hand-side.
*
* Query the status structure with *braid_StepStatusGetTstart(status, &tstart)*
* and *braid_StepStatusGetTstop(status, &tstop)* to get *tstart* and *tstop*.
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19 changes: 13 additions & 6 deletions docs/Examples.md
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Expand Up @@ -527,12 +527,19 @@ be implemented in some of the following examples.
at C-points in the default setting. When storage is turned on at F-points,
the better initial guess becomes available everywhere.

In general, the user should always use the *ustop* parameter in
*Step()* as the initial guess for an implicit solve. If storage
is turned on (i.e., set to 0), then this value will always be the improved
initial guess for C- and F-points. If storage is not turned on, then
this will be the improved guess only for C-points. For F-points,
it will equal the solution from the previous time step.
In general, the user should always use the *ustop* parameter in *Step()* as
the initial guess for an implicit solve. If storage is turned on (i.e., set
to 0), then this value will always be the improved initial guess from the
previous XBraid iteration for C- and F-points, at the cost of storing more
information. If storage is not turned on (i.e., set to the default -1),
then at

- Level 0, F-points, *ustop* = *u* in *Step()*, i.e., no improved information is available

- Level 0, C-points, *ustop* = the old solution value at *tstop* in *Step()*, i.e., improved information is available

- All coarse levels, *ustop* = the restricted fine-grid solution value (the
initial guess at tstop), i.e., improved information is available

See ``examples/ex-03`` for an example which uses this feature.

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