Plural of an initialism written in lowercase with abbreviation full stops
Although similar questions have been already posed and answered here, I still have this doubt: in the case of initialisms written in lowercase (and possibly having full stops after each letter), how should we write their plurals? Should we be using an apostrophe before the 's'; no apostrophe; no 's' at all...?
EXAMPLE 1: The term random variable is usually abbreviated as r.v. So, several random variables could be referred to as r.v.'s?? If not, how?
EXAMPLE 2: Cumulative distribution function is usually abbreviated as c.d.f.; or even cdf, very often.
EXAMPLE 3: Very famous in Maths: greatest common divisor is usually abbreviated as gcd.
This question differs from previous ones in that I am interested in the plural of initialisms / acronyms written in lowercase.
Solution 1:
One approach, from what seems an immaculate publication from San Jose State University › faculty › gerstman {B. Burt Gerstman, D.V.M., M.P.H., Ph.D. Professor Emeritus}:
4: Probability ...
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pdf: probability density function
pmf: probability mass function
RV: random variable
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[p 2] Binomial random variables are discrete RVs of “counts” that describe the number of “successes”
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[p 4] Probability mass functions (pmfs) can be drawn as pmf histograms.
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[p 5, bottom] For additional instruction on pdfs see §5.4 in Basic Biostatistics for Public Health Practice (Gerstman 2015, Jones & Bartlett, Burlington, MA).
(1) This is one suggestion; you can probably find others. From the next nearest educational establishment, perhaps, with equally immaculate publications. There are usually in-house specifications (which are not universally binding ... merely university-binding).
(2) Note that periods are not used here in initialisms (these are not acronyms in the most usually accepted default sense of the word, as individual letters are read out; contrast NATO); this is standard practice in the UK for all abbreviations where there are not compelling reasons to add a full stop, and is catching on quite rapidly in the States. Stan Carey at WordPress offers advice on cases where periods are opted for.
(3) Note that capitalisation seems to follow arbitrary tradition rather than strict logic.
(4) The 'rule' used here is 'simply add a lower-case s to pluralise any initialism/acronym, no matter what the case is.' I'd go with this myself, but would reserve the right to insert an apostrophe where I felt it would aid clarity.