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Re: Russian Peasant Multiplication: how does it work?
Posted:
Mar 3, 1998 2:47 AM


In article <34FAEB0F.C90@ufl.edu>, Kevin Fortin <kfortin@ufl.edu> wrote: >Hello, > >Could someone point me toward a good explanation of how "Russian Peasant >Multiplication" works? I saw the method in Jan Gullberg's recent book >"Mathematics: From the Birth of Numbers", but I still can't grasp why it >works.
This is really easy to understand on the basis of common sense. Unfortunately, most math books try to give a clever explanation based on the binary representation of numbers. This works, but the explanation is harder to follow.
Okay, suppose you want to multiply x by 16. Since 16 = 2^4, what you should do is to double x 4 times. Now for a larger number, you might not know quickly what the right exponent is. Say take 512, which is a power of 2, but you've forgotten which power. But just write down 512 and write down x. Now divide 512 by 2, getting 256, and at the same time double x. Keep dividing the lefthand column (the one that starts with 512) by 2 and doubling the numbers in the righthand column (which started with x), until you've reduced the lefthand column to 1. The number at the bottom of the righthand column is the product you're seeking. It's easy to see why this works, because as you move down both columns simultaneously, the product of the numbers in the two columns is always the same. Looking at the top entries, this product is clearly 512 times x. Looking at the bottom entry gives you the product.
But suppose you want to multiply by a number which is not a power of 2. Say I want to multiply x by 17. Then I go through the same process, except that when you divide the numbers in the lefthand column by 2, just ignore the remainder in case the number is odd. Now to obtain the final answer I need to add x to the number at the bottom of the righthand column. In this case, as we move down the two columns, the product at the second step is no longer the same as the product at the first, because 17 became 8, which is not actually half of 17.
In general, if you're multiplying x by a and go through this process, every time the number in the lefthand column is odd, the product of the two columns one line down will be smaller than the product before. To compensate, you need to add in the number which is in the righthand column at this stage when you get to the end. (Cross out all the rows where the lefthand column is even, but save the rest. Now add up all the entries in the righthand column which are not crossed out and you'll have your answer.)
If you want a longer explanation, with several examples, look at "Bride of the Lazy Man" at <Http://www.math.Hawaii.Edu/~lee/elementary/Lazy2.pdf>. (This file is also available in postscript, dvi, and idvi formats. Look at www.math.Hawaii.Edu/~lee/elementary/ and click on the format you prefer.)
 Trying to understand learning by studying schooling is rather like trying to understand sexuality by studying bordellos.  Mary Catherine Bateson, Peripheral Visions lady@Hawaii.Edu



