**Edited to be on-topic...
Some resources I've found state that the recapitulation presents the first and/or second subject groups (in any order) in the tonic key.
Today a colleague questioned the 'and/or', on the grounds that the first subject group is occasionally omitted in the recapitulation, but not the second. His perspective was that the second subject group (or at least part of it) must be restated in the tonic key, and doing so helps to 'resolve' the work, since this has only ever been restated in a different key. By contrast, the first subject has already been stated in the tonic key, and is therefore already 'resolved', and can be omitted.
At a glance, his advice checks out - I can find examples of recaps with either both subjects, or the second subject only, but not with the first subject only.
My question is: is it possible for a recapitulation to exclusively restate the first subject (group), without restating the second subject (group)?
**Should clarify that I would discount examples of monothematic sonata form, since there is no distinct second subject to omit!