grammar - Possessive and plural of "Series"

I'm looking at the financial definition of series: a group of stocks or options that have common characteristics. Source

How would I form the possessive and plural of this term? I'm guessing it is series' and series respectively.

Sample sentences, not sure of correctness:

He purchased one series.

He purchased multiple series.

This one series' characteristics are worth researching. <- This feels like it should be series's, but it looks so wrong.

These series' value is unmatched.

Answer

All correct but the last one.

If you are talking about one series:

This series' value is unmatched

If you are talking about multiple series:

These series' values are unmatched.

If you are talking about multiple series valued together:

The value of these series' is unmatched.