>The sigmatic aorist is the regular aorist of verbs forming an sk-present.
This claim has been made several times. It may be true of an individual
language, but it is certainly not true of PIE.
A search of Rix's indices showed that he lists 51 roots definitely
forming -ske presents.
Of these, 1 forms a reduplicated aorist.
22 form root aorists
5 form definite s-aorists
1 perhaps forms an s-aorist.
Only one root definitely forms both a ske-present and a s-aorist in the same
language. There are three others that might.
So Rix's indices do not support a claim that ske-presents go with s-aorists
in PIE.
The claim might well be true within a particular language, but if so, that
is an innovation internal to that language, and either the ske-presents, or
the s-aorists, or both, will be recent formations.
A similar claim was made a while ago that nasal infix presents normally form
root aorists. I believe this was also shown to be untrue.
Peter