The Indian Sulbasutras
The Vedic people entered India about 1500 BC from the region that today is Iran. The word Vedic describes the religion of these people and the name comes from their collections of sacred texts known as the Vedas. The texts date from about the 15th to the 5th century BC and were used for sacrificial rites which were the main feature of the religion.
For the gods to be pleased everything had to be carried out with a very precise formula, so mathematical accuracy was seen to be of the utmost importance. We should also note that there were two types of sacrificial rites, one being a large public gathering while the other was a small family affair. Different types of altars were necessary for the two different types of ceremony.
This in itself gives us a problem, for we do not know if these people undertook mathematical investigations for their own sake, as for example the ancient Greeks did, or whether they only studied mathematics to solve problems necessary for their religious rites.
We shall look at both of these examples below but the point we wish to make here is that the Sulbasutras make no distinction between the two. Did the writers of the Sulbasutras know which methods were exact and which were approximations?
The Sulbasutras were written by a scribe, although he was not the type of scribe who merely makes a copy of an existing document but one who put in considerable content and all the mathematical results may have been due to these scribes.
We know nothing of the men who wrote the [[Sulbasutras] other than their names and a rough indication of the period in which they lived. Like many ancient mathematicians our only knowledge of them is their writings.
The most important of these documents are the Baudhayana Sulbasutra written about 800 BC and the Apastamba Sulbasutra written about 600 BC. Historians of mathematics have also studied and written about other Sulbasutras of lesser importance such as the Manava Sulbasutra written about 750 BC and the Katyayana Sulbasutra written about 200 BC.
Let us now examine some of the mathematics contained within the Sulbasutras. The first result which was clearly known to the authors is Pythagoras's theorem. The Baudhayana Sulbasutra gives only a special case of the theorem explicitly:-
The rope which is stretched across the diagonal of a square produces an area double the size of the original square.
The rope which is stretched along the length of the diagonal of a rectangle produces an area which the vertical and horizontal sides make together.
The diagram on the right illustrates this result.
While thinking of explicit statements of Pythagoras's theorem, we should note that as it is used frequently there are many examples of Pythagorean triples in the Sulbasutras. For example (5, 12, 13), (12, 16, 20), (8, 15, 17), (15, 20, 25), (12, 35, 37), (15, 36, 39), (5/2 , 6, 13/2), and (15/2 , 10, 25/2) all occur.
Now the Sulbasutras are really construction manuals for geometric shapes such as squares, circles, rectangles, etc. and we illustrate this with some examples.
The first construction we examine occurs in most of the different Sulbasutras. It is a construction, based on Pythagoras's theorem, for making a square equal in area to two given unequal squares. Consider the diagram on the right.
ABCD and PQRS are the two given squares. Mark a point X on PQ so that PX is equal to AB. Then the square on SX has area equal to the sum of the areas of the squares ABCD and PQRS. This follows from Pythagoras's theorem since SX2 = PX2 + PS2.
Consider the diagram on the right.
The rectangle ABCD is given. Let L be marked on AD so that AL = AB. Then complete the square ABML. Now bisect LD at X and divide the rectangle LMCD into two equal rectangles with the line XY. Now move the rectangle XYCD to the position MBQN. Complete the square AQPX.
Now the square we have just constructed is not the one we require and a little more work is needed to complete the work. Rotate PQ about Q so that it touches BY at R. Then QP = QR and we see that this is an ideal "rope" construction. Now draw RE parallel to YP and complete the square QEFG. This is the required square equal to the given rectangle ABCD.
EQ2 = QR2 - RE2
= QP2 - YP2
= ABYX + BQNM
= ABYX + XYCD
All the Sulbasutras contain a method to square the circle. It is an approximate method based on constructing a square of side 13/15 times the diameter of the given circle as in the diagram on the right.
This corresponds to taking π = 4 × (13/15)2 = 676/225 = 3.00444 so it is not a very good approximation and certainly not as good as was known earlier to the Babylonians.
It is worth noting that many different values of π appear in the Sulbasutras, even several different ones in the same text. This is not surprising for whenever an approximate construction is given some value of π is implied.
The authors thought in terms of approximate constructions, not in terms of exact constructions with π but only having an approximate value for it.
For example in the Baudhayana Sulbasutra, as well as the value of 676/225, there appears 900/289 and 1156/361. In different Sulbasutras the values 2.99, 3.00, 3.004, 3.029, 3.047, 3.088, 3.1141, 3.16049 and 3.2022 can all be found; see . In  the value π = 25/8 = 3.125 is found in the Manava Sulbasutras.
In  in addition to examining the problem of squaring the circle as given by Apastamba, the authors examine the problem of dividing a segment into seven equal parts which occurs in the same Sulbasutra.
Consider the diagram on the right.
The following construction appears. Given a square ABCD find the centre O. Rotate OD to position OE where OE passes through the midpoint P of the side of the square DC. Let Q be the point on PE such that PQ is one third of PE. The required circle has centre O and radius OQ.
r = OE - EQ
= √2a - 2/3(√2a - a)
= a (√2/3 + 2/3).
Then 4a 2 = πa2 (√2/3 + 2/3)2
which gives π = 36/(√2 + 2)2 = 3.088.
Increase a unit length by its third and this third by its own fourth less the thirty-fourth part of that fourth.
Now this gives
√2 = 1 + 1/3 + 1/(3 × 4) - 1/(3 × 4 × 34) = 577/408
which is, to nine places, 1.414215686. Compare the correct value √2 = 1.414213562 to see that the Apastamba Sulbasutra has the answer correct to five decimal places. Of course no indication is given as to how the authors of the Sulbasutras achieved this remarkable result. Datta, in 1932, made a beautiful suggestion as to how this approximation may have been reached.
In  Datta considers a diagram similar to the one on the right.
The most likely reason for the construction was to build an altar twice the size of one already built. Datta's suggestion involves taking two squares and cutting up the second square and assembling it around the first square to give a square twice the size, thus having side √2.
The second square is cut into three equal strips, and strips 1 and 2 placed around the first square as indicated in the diagram.
The third strip has a square cut off the top and placed in position 3. We now have a new square but some of the second square remains and still has to be assembled around the first.
Cut the remaining parts (two-thirds of a strip) into eight equal strips and arrange them around the square we are constructing as in the diagram. We have now used all the parts of the second square but the new figure we have constructed is not quite a square having a small square corner missing. It is worth seeing what the side of this "not quite a square" is. It is
1 + 1/3 + 1/(3 × 4)
which, of course, is the first three terms of the approximation. Now Datta argues in  that to improve the "not quite a square" the Sulbasutra authors could have calculated how broad a strip one needs to cut off the left hand side and bottom to fill in the missing part which has area (1/12)2. If x is the width one cuts off then
- 2 × x × (1 + 1/3 + 1/12) = (1/12)2.
This has the solution x = 1/(3 × 4 × 34) which is approximately 0.002450980392. We now have a square the length of whose sides is
- 1 + 1/3 + 1/(3 × 4) - 1/(3 × 4 × 34)
Of course we have still made an approximation since the two strips of breadth x which we cut off overlapped by a square of side x in the bottom left hand corner. If we had taken this into account we would have obtained the equation
- 2 × x × (1 + 1/3 + 1/12) - x2 = (1/12)2
for x which leads to x = 17/12 - √2 which is approximately equal to 0.002453105. Of course we cannot take this route since we have arrived back at a value for x which involves √2 which is the quantity we are trying to approximate!
In  Gupta gives a simpler way of obtaining the approximation for √2 than that given by Datta in . He uses linear interpolation to obtain the first two terms, he then corrects the two terms so obtaining the third term, then correcting the three terms obtaining the fourth term.
Although the method given by Gupta is simpler (and an interesting contribution) there is certainly something appealing in Datta's argument and somehow a feeling that this is in the spirit of the Sulbasutras.
If we follow the suggestion of some historians that the writers of the Sulbasutras were merely copying an approximation already known to the Babylonians then we might come to the conclusion that Indian mathematics of this period was far less advanced than if we follow Datta's suggestion
1. B Datta, The science of the Sulba (Calcutta, 1932).
2. G G Joseph, The crest of the peacock (London, 1991).
3. R C Gupta, New Indian values of π from the Manava sulba sutra, Centaurus 31 (2) (1988), 114-125.
4. R C Gupta, Baudhayana's value of √2, Math. Education 6 (1972), B77-B79.
5. S C Kak, Three old Indian values of π, Indian J. Hist. Sci. 32 (4) (1997), 307-314.
6. R P Kulkarni, The value of π known to Sulbasutrakaras, Indian J. Hist. Sci. 13 (1) (1978), 32-41.
7. G Kumari, Some significant results of algebra of pre-Aryabhata era, Math. Ed. (Siwan) 14 (1) (1980), B5-B13.
8. A Mukhopadhyay and M R Adhikari, The concept of cyclic quadrilaterals : its origin and development in India (from the age of Sulba Sutras to Bhaskara I, Indian J. Hist. Sci. 32 (1) (1997), 53-68.
9. A E Raik and V N Ilin, A reconstruction of the solution of certain problems from the Apastamba Sulbasutra of Apastamba (Russian), in A P Juskevic, S S Demidov, F A Medvedev and E I Slavutin, Studies in the history of mathematics 19 "Nauka" (Moscow, 1974), 220-222; 302.
Article by: J J O'Connor and E F Robertson