### Romanorum computus

"...This is the conventional wisdom concerning Roman arithmetic, butit is easy to understand that although addition of Roman numerals is quite satisfactory, multiplication and division are essentially impossible." - An overview of Egyptian mathematics

**it can not be correct**since the Romans were great engineers and administrators of a large empire, and these tasks are impossible without efficient methods of multiplication and division. In fact, the previous great empire of Egypt had efficient methods prior to 1850 BCE - a fact known from the Rhind papyrus which is the earliest extant description of their arithmetical methods. It is likely that the Greeks and Romans were aware of these methods and adopted some form of them, since the Egyptian numerals were similar to the later Roman numerals.

Symbol | Value |

I | 1 (one) (unus) |

V | 5 (five) (quinque) |

X | 10 (ten) (decem) |

L | 50 (fifty) (quinquaginta) |

C | 100 (one hundred) (centum) |

D | 500 (five hundred) (quingenti) |

M | 1000 (one thousand) (mille) |

CXVI (116) + XXIIII (24) = CXVIXXIIII = CXXXVIIIII = CXXXX (140)

where the base ten values are show in parenthesis, and the group VIIIII has been replaced by X. Subtraction is not much more difficult: eliminate common symbols in both the minuend and subtrahend (indicated below by the strike-thrus), expand symbols in the minuend as necessary to produce the remaining symbols in the subtrahend, complete the elimination, and combine any possible group of symbols, e.g.:

DCCXXVIII (728) - LI (51) = DCCXXVII

It is evident that these processes are applicable to any numbers, no matter how large.

Neither of these operations provides any clue as to how multiplication and division could be done. It was certainly not done by the algorithms here, for the number of additions or subtractions for any large multiplier is prohibitive. A Different Kind of Multiplication suggests that they used a form of peasant multiplication.

(1) | (2) | |||

XXVII | (27) | * | LXXXII | (82) |

XIII | (13) | * | CLXIIII | (164) |

VI | (6) | CCCXXVIII | (328) | |

III | (3) | * | DCLVI | (656) |

I | (1) | * | MCCCXII | (1312) |

LXXXII+CLXIIII+DCLVI+MCCCXII = MDCCCCCLLLXXXXXVIIIIIIIII = MMCCXIIII (2214)

That this is the correct answer can be easily verified by the reader, and the labor involved is only slightly more than the modern manual method of multiplication using arabic numerals. Dr. David P. Stern, the author of A Different Kind of Multiplication, opines:

"It was probably discovered by trial and error, and it always worked, though the Romans did not know why."That they didn't know why is probably true, but it is more likely that they learned the method from some other group - perhaps the Greeks - though not the Egyptians, because they used a slightly different method discussed below.

The reason it always works is as follows. Many will recognize that repeated division by 2, keeping track of the remainders, is the method to convert any number to base-2 notation. If the number is even, there is no remainder (i.e., a remainder of 0), while if odd there is a remainder of 1. Reading from the bottom up in column one of the previous table and writing a 1 for the odd numbers and 0 for the even:

11011_{2} = 1x2^{4}+1x2^{3}+1x2^{1}+1x2^{0} = 16+8+2+1 = 27

we see that 11011_{2} is the correct base-2 representation of 27. If we now multiply by 82:

82x(1x2^{4}+1x2^{3}+1x2^{1}+1x2^{0}) = 82x2^{4}+82x2^{3}+82x2^{1}+82x2^{0}

because multiplication is distributive. Now:

82x2^{4} = 82x16 = 1312

82x2^{3} = 82x8 = 656

82x2^{1} = 82x2 = 164

82x2^{0} = 82x1 = 82

It is clear that the doubling in column two produces precisely the power of 2 factors that when added together give the correct result. Unfortunately, this does not explain how the Romans would have divided.

(1) | (2) | |

1 | * | 82 |

2 | * | 164 |

4 | 328 | |

8 | * | 656 |

16 | * | 1312 |

32 | 2624 |

As to what method the Romans actually used, the answer is no one knows for sure. The age of the peasant method and the presence of the 5 factor symbols in the Roman numerals, facilitating halving and doubling, argues for the use of the peasant method, but leaves the explanation of division unresolved. What is amazing is the fact that nearly 4,000 or more years ago some unknown mathematical genius discovered that any number can be represented as a sum of powers of two, that multiplication is distributive, and combined these insights into efficient algorithms for multiplication and division, reducing them to the simpler operations of doubling, adding, and subtracting.

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