## Tuesday, October 17, 2017

### Romans and multiplication

Roman Numericals and Arithmetic “Multiplication using roman numerals is not particularly easy or obvious. You can do the trivial thing, which is repeated addition. But it should be pretty obvious that that’s not practical for large numbers. The trick that they used was actually pretty nifty. It’s basically a strange version of binary multiplication. You need to be able to add and divide by two, but those are both pretty easy things to do. So here goes:
.....
Division is the biggest problem in roman numerals. There is no good trick that works in general. It really comes down to repeated subtraction. The only thing you can do to simplify is variations on finding a common factor of both numbers that’s easy to factor out. For example, if both numbers are even, you can divide each of them by two before starting the repeated subtraction. It’s also fairly easy to recognize when both numbers are multiples of 5 or 10, and to do the division by 5 or 10 on both numbers. But beyond that, you take a guess, do the multiplication, subtract, repeat.“