Bombelli: Algebra
Many methods of forming fractions have been given in the works of other authors; the one attacking and accusing the other without due cause (in my opinion) for they are all looking at the same end. It is indeed true that one method may be briefer than another, but it is enough that all are at hand and the one that is most easy will without doubt be accepted by men and be put in use without casting aspersions on another method. ... In short, I shall set forth the method which is most pleasing to me today and it will rest in men's judgement to appraise what they see; meanwhile I shall continue my discourse going now to the discussion itself.
Let us first assume that if we wish to find the approximate root of 13 that this will be 3 with 4 left over. The remainder should be divided by 6 (double the 3 above) which gives ^{2}/_{3} . This is the first fraction to be added to the 3, making 3 ^{2}/_{3} which is the approximate root of 13. Since the square of this number is 13 ^{4}/_{9} , it is ^{4}/_{9} too large, and if one wishes a closer approximation, the 6, which is double the 3, should be added to the fraction ^{2}/_{3} giving 6 ^{2}/_{3} and this number should be divided into 4 which is the difference between 13 and 9 ...
Let us suppose we are required to find the root of 13. The nearest square is 9, which has root 3. I let the approximate root of 13 be 3 plus 1 unknown.
With some modern tools, let us investigate what Bombelli is doing. Here is Maple code for Bombelli's recursive procedure
bombelli:= proc(n)
This gives the first 10 fractions as:
option remember; if n = 0 then 0 else 4/(6+bombelli(n - 1)) end if; end proc: ^{2}/_{3} , ^{3}/_{5} , ^{20}/_{33} , ^{66}/_{109} , ^{109}/_{180} , ^{720}/_{1189} , ^{2378}/_{3927} , ^{3927}/_{6485} , ^{25940}/_{42837} , ^{85674}/_{141481} The last term as a decimal correct to 12 places is .605551275436 Compare this with the fractional part of √13: .605551275464 Of course we can also see that Bombelli's procedure leads to a continued fraction expansion of a square root. Cataldi's work on continued fractions appeared in 1613, well after Bombelli's work of 1572. Since Bombelli is not thinking in terms of continued fractions it would not be appropriate to claim that he was the inventor of continued fractions because of the above work. however, it is quite possible that Cataldi had the idea of a continued fraction after reading Bombelli's book. |
JOC/EFR August 2006
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