Rules of Roman Numerals

Question

I've been working on a problem that involves discovering valid methods of expressing natural numbers as Roman Numerals, and I came across a few oddities in the numbering system.

For example, the number 5 could be most succinctly expressed as $\texttt{V}$, but as per the rules I've seen online, could also be expressed as $\texttt{IVI}$.

Are there any rules that bar the second expression from being valid? Or are the rules for roman numerals such that multiple valid expressions express the same number.

Edit

A sample set of rules I've seen online:

1. Only one I, X, and C can be used as the leading numeral in part of a subtractive pair.
2. I can only be placed before V or X in a subtractive pair.
3. X can only be placed before L or C in a subtractive pair.
4. C can only be placed before D or M in a subtractive pair.
5. Other than subtractive pairs, numerals must be in descending order (meaning that if you drop the first term of each subtractive pair, then the numerals will be in descending order).
6. M, C, and X cannot be equalled or exceeded by smaller denominations.
7. D, L, and V can each only appear once.
8. Only M can be repeated 4 or more times.

Answer 1

Please keep in mind that strict rules for Roman numerals were established in a recent time. Besides having different signs for the digits, subtractive system was not fixed. In an inscription found in Forum Popilii and dated between 172 BC and 158 BC, 84 is written as XXCIIII (quoted from Georges Ifrah, The Universal History of Numbers: I have the Italian edition, so I cannot provide the page number for the English edition).

As per your question, subtractive and addictive rule cannot be applied to the same number (V, in your case). IIII and XXXX are widely used too: subtractive rule was not mandatory, but rather used as a shortcut.

Answer 2

"Roman numerals" presumably means how the actual Romans actually wrote down numbers.

They would never have written five as IVI, full stop.

If you're following a particular set of formal rules that are not "do things as the actual Romans would have done them", then follow those rules. Be prepared to get nonsense results if the rules you follow are sloppily phrased.

Answer 3

Wikipedia explicitly enumerates the patterns for the units, the tens and the hundredths.

https://en.wikipedia.org/wiki/Roman_numerals#Basic_decimal_pattern

This doesn't leave room for extravagant expressions, though some variants are described. Subtractive-additive forms were of course not used.