What is the proper plural of “a series”?

Series (like deer, salmon, and sheep) is pronounced and spelled the same in the plural as in the singular. If either the singular or the plural is used as a possessive, an apostrophe is added to show that in print, though there is no pronunciation difference in speech. Thus,

• The series doesn't converge. (singular)
• The series don't converge. (plural)
• This series' limit is unknown. (singular possessive)
• These series' limits are unknown. (plural possessive)

More examples can be seen here.