The way we write out Roman numerals today differs in many respects to how the ancient Romans actually did themselves and their rules were 4 times for I, only once for V, 4 times for X, only once for L, 4 times for C, only once for D and so the pattern continues.
The reason being: IIII = 4 but IIIII = V = 5 and V+V = X =10
There are 3 symbols that are not repeated when forming numerals. They are V, L, and D (5, 50, and 500).
The symbol "D" can never be repeated in Roman numerals. According to the traditional rules of Roman numerals, "D" represents the number 500. It cannot be repeated to represent larger numbers, such as 1000 or 5000.
You have already given it in Roman numerals
In Roman numerals, MMMD is MMMD. In Hindu-Arabic numerals, it is 3500.
They are meaningless because they are an invalid arrangement of Roman numerals
The numerals: V, L and D are only used once in a row The numerals: I, X, C and M are only used fourfold in a row
In Roman numerals, M represents the value 1000. There is no limit to how many times M can be repeated in a Roman numeral representation, so it can appear as many times as needed to represent the desired value. For example, MMM represents 3000.
Under today's rules the Roman numerals CMXLVII represent 947.
It is nothing. That is not a roman numeral!
399
190
1,915