ब्राह्मस्फुटसिद्धान्त (ब्रह्मगुप्त - शून्य, कुट्टक, बीजगणित एवं सम्पूर्ण २१ अध्याय सान्वय सटीक)

238- BRAHMAGUPTA AS AN ALGEBRAIST cular case only of the more general latter equation. Of course, there is a little justification also for treating it separately, since both the types of equations represent two different physical conditions of the astronomical problems. In the case of by=ax±c, the conditions are such that the value of either y or x, more particu- larly of the latter, has to be found and the rules for solution formulated with that objective. But in the case of the equation by=ax±1, the physical conditions require the values of both y and x. The equation by=ax±1 is usually known by the name sthira-kuṭṭaka, literally meaning the 'constant pulveriser' Pṛthū- daka Svāmī also names it as dṛḍha-kuṭṭaka meaning firm-pulveri- ser. Later on this term dṛḍha-was confined to another sense, equivlent to nicched (having no divisor) or nirapavarta (irreduci- ble). The origin of the name sthira-kuṭṭaka or constant pulveriser has been explained by Pṛthūdaka Svāmī as being due to the fact that the interpolator (±1) is here invariable. For the solution of this equation, we shall quote Bhāskara I's rule and the rule by Brahmagupta. Bhāskara I writes in this connection as follows : The method of the pulveriser is applied also after subtracting unity. The multiplier and quotient are respectively the numbers above and underneath. Multi- plying those quantities by the desired number divide by the reduced divisor and dividend; the residues are in this case known to be the (elapsed) days and (resid- ues of) revolutions respectively¹. The pulveriser ax - c -------- = y ... (1) b may be written as aX - 1 -------- = Y ... (2) b where x=cX and y=cY. If X=α, Y=β is a solution of (2), then x=cα, y=cβ will be a solution of (1). Hence the above rule.

1. रूपमेकमपास्यापि कुट्टाकारः प्रसाध्यते । गुणकारोऽथ लब्धं च राशी स्यातामुपर्यधः ॥ —MBh. I. 45

BRAHMAGUPTA'S RULES OF ANALYSIS 239 Brahmagupta's Rule in this connection is as follows : Solution of by=ax-1 : Divide them (i.e., the abraded coefficient of the multi- plier and the divisor) mutually and set down the quo- tients one below the other. The last residue (or the reciprocal division after an even number of quotients has been obtained) is multiplied by an optional integer such that the product being diminished by unity will be exactly divisible (by the divisor corresponding to that residue). The (optional) multiplier and then this quotient should be set down (underneath the listed quotients). Now proceeding from the lower most term to the uppermost, by the penultimate multiply the term just above it and then add the lowermost number. (The uppermost number thus calculated being divided by the reduced divisor, the residue (is the quan- tity required. This is the method of the constant pul- veriser¹. Solution of by+ax=±c Indian algebraists usually transformed this equation as by=-ax+c, so that it appeared as a particular case of by=ax+c, in which a was negative. Brahmagupta has been the first person to solve this equation, but the rule given by him is obscure : The reversal of the negative and positive should be made of the multiplier and interpolator.² Pṛthudaka Svāmī has tried to explain it, but he too is not very clear. He says :

1. हृतयोः परस्परं यच्छ्रेषं गुणकार भागहारकयोः । तेन हृतौ निश्छेदौ तावेव परस्परं हृतयोः ॥ लब्धमधोऽधः स्थाप्यं तथेष्ट गुणकारसङ्गुणं शेषम् । शुद्धस्यति यथैकहीनं गुणकः स्थाप्यः फलं चान्त्यम् ॥ अश्रान्तमुपान्त्येन स्वोर्ध्वो गुणितोऽन्त्य संयुतो भक्तम् । निःशेषभागहारेणैव स्थिरकुट्टकः शेषम् ॥ —BrSpSi. XVIII. 9-11
2. एवं समेषु विषमेष्वृणं धनं धनमृणं यदुक्तं तत् । ऋणधनयोर्व्यस्तत्वं गुण्यप्रक्षेपयोः कार्यम् ॥ BrSpSi XVIII. 13

240 BRAHMAGUPTA AS AN ALGEBRAIST If the multiplier be negative, it must be made positive; and the additive must be made negative : and then the method of the pulveriser should be employed. Pṛthūdaka Svāmī, however, does not indicate how to derive the solution of the equation. by = -ax+c ...(1) from that of the equation by = ax-c ...(2) The method, however, seems to have been this : Let x=α, y=β be the minimum solution of (2). Then we get bβ = a α-c or b(a-β) = -a(a-b)+c Hence x=a-b, y=a-β is the minimum solution of (1). This rule is very clearly indicated by Bhāskara II and others. We shall give two examples from Bhāskara II (Bījagaṇita) to illustrate the rule : Example I. 13y = -60x + 3 By the method described before, we find that the minimum solution of 13y = 60x+3 is x=11, y=51. Subtracting these values from their respective abraders, namely 13 and 60, we get 2 and 9. Then by the maxim : "In the case of the dividend and divisor being of differ- ent signs, the results from the operation of division should be known to be so", making the quotient negative we get the solu- tion of 13y = -60x+3 as x=2, y=-9. Subtracting these values again from their respec- tive abraders (13, 60), we get the solution of 13y = -60x-3 as x=11, y=-51. Example II. 11y = 18x+10

LINEAR EQUATIONS 241 Proceeding as before, we find the minimum solution of 11y=18x+10 to be x=8, y=14. These will also be the values of x and y in the case of the negative divisor but the quotient for the reasons stated before should be made negative. So the solution of -11y=18x+10 is x=8, y=-14. Subtracting these (i.e., their numerical values) from their respective abraders, we get the solution of -11y=18x-10 as x=3, y=-4, "When the divisor is positive or negative the numeri- cal values of the quotient and multiplier remain the same : when either the divisor or the dividend is negative, the quotient must always be known to be negative"¹. One Linear Equation in More Than Two Unknowns Whenever a linear equation involves more than two unkno- wn's the Indian algebraists used to assume arbitrary values for all the unknowns except two and then to apply the method of kuṭ- ṭaka or "pulveriser". In this connection, Brahmagupta says : The method of the pulveriser (should be employed if there be present many unknowns (in any equation)²,

1. Bhāskara II gives the following rule : "Those(the multiplier and quotient)cbtained for a positive divi- dend being treated in the same manner give the results corres- ponding to a negative dividend." The treatment alluded to in this rule is that of subtraction from the respective abraders. He has further elaborated it thus : The multiplier and quotient should be determined by taking the dividend, divisor and interpolator as positive. They will be the quantities for the additive interpolator. Subtracting them from their respective abraders, the quantities for a negative interpolator are found. If the dividend or divi- sor, be negative, the quotient should be stated as negative, the quotient should be stated as negative. —Bījagaṇita
2. आद्याद्वर्णादन्यान् वर्णान् प्रोह्याद्यमानमाद्यहृतम् । सदृशच्छेदावसकृद् द्वौ व्यस्तौ कुट्टको बहुषु ॥ — BrSpSi. XVIII. 51

242 BRAHMAGUPTA AS AN ALGEBRAIST We shall take up one of the problems posed by Brahmagupta concerning astronomy and leading to the equation :¹ 197x - 1644 y - z = 6302. Hence 1644 y + z + 6302 x = ------------------- 197 The commentator assumes z = 131. Then 1644 y + 6433 x = --------------- ; 197 hence by the usual method of the pulveriser x = 41; y = 1. General Problem of Remainders A certain type of simultaneous indeterminate equations of the first degree arise out of the general problem of remainders which may thus be stated : To find a number N which being severally divided by a₁, a₂, a₃.......aₙ , leaves as remainders r₁, r₂, r₃.........rₙ respectively. While dealing with such a case, we shall have the following series of equations : N = a₁x₁ + r₁ = a₂x₂ + r₂ = a₃x₃ + r₃ = ...... = aₙ xₙ + r . We have reasons to believe that the method of solution of these equations was known to Āryabhaṭa I. In the translation of the verse in the Āryabhaṭīya, II. 32-33 (the translation of which we have already given), the term dvicchedāgram should be translated as "the result will be the remainder corresponding to the product of the two divisors", instead of "the result will be the number corresponding to the two divisors." (the last line of the translation). This explanation is in fact given by Bhāskara I, the direct disciple and earliest commentator of Āryabhaṭa I. Such a rule is clearly stated by Brahmagupta².

1: अंशकशेषेण युतात् लिप्ताशेषात्तदन्तरादथवा । भानोर्भ'दिने द्युगणं यः कथयति कुट्टकज्ञः सः ॥ —BrSpSi. XVIII. 55 2: स्वोच्छेऽन्ययुतोऽग्रान्तो हीनाग्रच्छेदभाजितः शेषम् । अधिकाग्रच्छेदहतमधिकाग्रयुतं भवत्यग्रम् ॥ —BrSpSi. XVIII. 5

GENERAL PROBLEM OF REMAINDERS 243 The rationale of this method is not difficult. I shall quote it from the book of Datta and Singh: Starting with the considera- tion of the first two divisors, we have N = a₁x₁ + r₁ = a₂x₂ + r₂. By the method described before, we can find the minimum value α of x₁ satisfying this equation. Then the minimum value of N will be a₁α + r₁. Hence the general value of N will be given by N = a₁ (a₂t + α) + r₁ = a₁a₂ t + a₁α + r₁ where t is an integer. Thus a₁α + r₁ is the remainder left on dividing N by a₁ a₂ as stated by Āryabhaṭa I and Brahmagupta. Now taking into consideration the third condition, we have N = a₁a₂t + a₁αr₁ = a₃x₃ + r₃ which can be solved in the same way as before. Proceeding in this way successively, we shall ultimately arrive at a value of N satisfying all the conditions ; Pṛthūdaka Svāmī remarks : Wherever the reduction of two divisors by a common measure is possible, there 'the product of the divisors' should be understood as equivalent to the product of the divisor corresponding to the greater remainder and quotient of the divisor corresponding to the smaller remainder as reduced (i.e. divided) by the common mea- sure.¹ When one divisor is exactly divisible by the other, then the greater remainder is the (required) remainder and the divisor corresponding to the greater remainder is taken as 'the product of the divisors'. (The truth of) this may be investigated by an intelligent mathematician by taking several symbols. As an illustration we shall take up a problem quoted by Bhāskara II in his Bījagaṇita, and which in its solution follows the method of Āryabhaṭa 1. Pṛthūdaka Svāmī while commenting on serveral verses from Brahmagupta (BrSpSi. XVIII. 3-6)

1. i.e., if p be the L.C.M. of a, and a2, the general value of N satisfying the above two conditions will be N = pt + a₁a + r, instead of N = a₁a₂t + a₁a + r₁.

244 BRAHMAGUPTA AS AN ALGEBRAIST observes that such problems were very popular amongst the an- cient Indian mathematicians. Problem : To find a number N which leaves remainders 5, 4,3,2 when divided by 6,5,4,3 respectively. That is to solve the equations : N = 6x + 5 = 5y + 4 = 4z + 3 = 3w + 2. We have since N = 6x + 5 = 5y + 4, x = (5y - 1) / 6 But x must be integral, so y = 6t + 5, x = 5t + 4 Hence N = 30t + 29 Again N = 30t + 29 = 4z + 3 Therefore, t = (2z - 13) / 15 Since t must be integral, we must have z = 15s + 14; hence t = 2s + 1. Therefore N = 60s + 59. The last condition is identically satisfied. The method given here is the one followed by Pṛthūdaka Svāmī. Thus when N = 60s + 59 = 6x + 5 x = (60s + 54) / 6 = 10s + 9 ...(1) Again, when N = 60s + 59 = 5y + 4, y = (60s + 55) / 5 = 12s + 11 Again when N = 60s + 59 = 4z + 3 z = (60s + 56) / 4 = 15s + 14 Lastly, when N = 60s + 59 = 3w + 2. w = (60s + 57) / 3 = 20s + 19, Varga Prakṛti or Kṛti Prakṛti or Square-Nature The word varga-prakṛti (literally meaning 'square-nature') has been given by Indian algebraists to the indeterminate quadra- tic equation Nx² ± c = y²

VARGA PRAKṚTI 245 Here in this equation the absolute number c should be rūpa (or unity). which means the equation Nx²±1=y² or it may be any absolute number. The most fundamental equation of this class has been regarded as Nx²+1=y² where N is a non-square integer. This branch of mathematics has originated from the number which is the prakṛti of the square of yāvat, etc. (the unknown x etc.), and therefore, it is called varga-prakṛti. The quantity N of the above equation is known as Prakṛti. Brahmagupta uses the term GUṆAKA (multiplier) for the same purpose¹. This term guṇaka together with its variation guṇa appears occasionally also in the writings of later authors. For example, Śrīpati (Siddhānta-śekhara. XIV. 32) employs the term guṇaka where as Bhāskara II and Nārāyaṇa use the term guṇa in their Bījagaṇitas. In this connection, we would now like to quote from Pṛthūdaka Svāmī (863 A.D.) from his commentary on the Brāhmasphuṭasiddhānta : Here are stated for ordinary use the terms which are well known to people.. The number whose square, multiplied by an optional multiplier and then increased or decreased by another optional number, becomes capable of yielding a square-root, is designated by the term the "lesser root" kaniṣṭha pada or the "first root" ādya-mūla). The root which results, after those opera- tions have been performed is called by the name the "greater root" (jyeṣṭha pada) or the "second root" (anya-mūla). If there be a number multiplying both these roots, it is called the "augmenter" (udvartaka); and on the contrary, if there be a number dividing the roots, it is called the "abridger" (apavartaka), Thus in the equation Nx²±c=y²,

1. मूलं द्विधेष्ट वर्गाद् गुणक गुणादिष्टयुत विहीनाच्च । आद्यवधो गुणकगुणः सहान्त्यघातेन कृतमन्त्यम् ॥ —BrSpSi. XVIII. 64
2. BrSpSi. XVIII. 64 (Com.)

246 BRAHMAGUPTA AS AN ALGEBRAIST x is known as the lesser root, y is the greater root, N is the multi- plier (guṇaka) and c is interpolator or kṣepaka. Bhāskara II has used the word "hrasvamūla" for kaniṣṭha pada or ādya-mūla lite- rally meaning "lesser root". The earlier terms, the "first root" (ādyamūla) for the value of x and the "second root" or the "last root" antya-mūla for the value of y are quite free from ambiguity Their use is found in the algebra of Brahmagupta. The later terms appears in the works of his commentator Pṛthūdaka Svāmī. Brahmagupta uses the term kṣepa, prakṣepa or prakṣepaka in the sense of "interpolator." Again, when negative, the inter- polator is sometimes distinguished as the "subtractive" or śodhaka and the positive interpolator is then called "the addi- tive." Lemmas of Brahmagupta Prior to our giving the general solution of the Square-nature or Varga-Prakṛti, it would be better to give two Lemmas esta- blished by Brahmagupta. We have the following in the Brāhma- sphuṭasiddhānta: Of the square of the optional number multiplied by the guṇaka and increased or decreased by an other optional number, iṣṭa, (extract) the square root. (Proceed) twice. The product of the first roots multiplied by the guṇaka together with the product of the second roots will give a (fresh) second root; the sum of their cross-pro- ducts will be a (fresh) first root. The (corresponding) interpolator will be equal to the product of the (previ- ous) interpolators.¹ There is a little difficulty in ascertaining the real sense of the rule given in these lines since the word dvidhā (twice) has two implications. Firstly, it may mean that the earlier operations of finding roots are made on two optional numbers with two optio- nal interpolators, and with the results thus obtained the subse-

1. मूलं द्विधेष्टवर्गाद् गुणक गुणादिष्टयुत विहीनाच्च । आद्यवधो गुणकगुणः सहान्त्यघातेन कृतमन्त्यम् ॥ वज्रवधैक्यं प्रथमं प्रक्षेपः क्षेपवध तुल्यः । प्रक्षेपशोधकहृते मूले प्रक्षेपके रूपे ॥ —BrSpSi. XVIII 64-65

LEMMAS OF BRAHMAGUPTA 247 quent operations of their composition are performed. Secondly, it may also mean that the earlier operations are made with one optionally chosen number and one interpolator, and the subsequent ones are carried out after the repeated statement of those roots for the second time. It is also implied that in the composition of the quadratic roots, their products may be added together or subtracted from each other. In other words, if x=a, y=β be a solution of the equation : Nx² + k = y², and x=a', y=β' be a solution of Nx² + k' = y², then according to the above x = aβ' ± a'β, y = ββ' ± Naa' is a solution of the equation Nx² + kk' = y². In other words, if Na² + k = β² Na'² + k' = β'² then · N(aβ' ± a'β)² + kk' = (ββ' ± Na')² (I) In particular, taking a=a', β=β' and k=k', Brahmagupta finds from a solution x=a, y=β of the equation Nx² + k² = y² a solution x=2aβ, y=β² + Na² of the equation Nx² + k = y² That is, if Na² + k = β² then N(2aβ)² + k² = (β² + Na²)² (II) This result will be hereafter called Brahmagupta's Corol- lary. Thus Brahmagupta's First Lemma says that if two solutions of the equation (of the Square-nature) Nx² + 1 = y² are known, then any number of other solutions can be found. For example if two solutions of the Square-nature are (a, b) and also (a', b'), then two other solutions will be : x = ab' ± a'b, y = bb' ± Naa'.

248 BRAHMAGUPTA AS AN ALGEBRAIST We can compose this solution with the previous ones, and get another solution, and thus proceed on to innumerable solu- tions. From Brahmagupta's Corollary to First Lemma we get another set of solutions. If (a, b) be solution of the Square- nature, then another solution of it is x=2ab, and y=b²+Na² Thus even if we have only one solution, we can get the other solution also (since N is known), and thus we can get any number of solutions one after the other by this Principle of Composition. Brahmagupta's Lemmas have been described by Bhāskara II (1150 A.D.) in the following words : Set down successively the lesser root (hrasva), greater root (jyeṣṭha) and interpolator (kṣepaka); and below them should be set down in order the same or an another (set of similar quantities). From them by the Principle of Composition (Bhāvanā) can be obtained numerous roots. Therefore the Principle of Composi- tion will be explained here. (Find), the two cross-pro- ducts (vajrābhyāsa) of the two lesser and the two greater roots; their sum is a lesser root. Add the product of the two lesser roots multiplied by the prakṛti to the product of the two greater roots, the sum will be a greater root. In that (equation) the interpolator will be the product of the two previous interpolators. Again the difference of the two cross-products is a lesser root. Subtract the product of the two lesser roots multiplied by the prakṛti from the product of the two greater roots; (the difference) will be greater root. Here also the interpolator is the product of the two (previous) interpolators.¹

1. ह्रस्वज्येष्ठ क्षेपकान् न्यस्य तेषां तानन्यान् वाऽधो निवेश्य क्रमेण । साध्यान्येभ्यो भावनाभिर्बहूनि मूलान्येषां भावना प्रोच्यतेऽतः ।। वज्राभ्यासौ ज्येष्ठलघ्वोस्तदैक्यं ह्रस्वं लघ्वोराहतिश्च प्रकृत्या । क्षुण्णा ज्येष्ठाभ्यास युग् ज्येष्ठमूलं तत्राभ्यासः क्षेपयोः क्षेपकः स्यात् ।। ह्रस्वं वज्राभ्यासयोरन्तरं वा लघ्वोर्घातो यः प्रकृत्या विनिघ्नः । घातो यश्च ज्येष्ठयोस्तद्वियोगो ज्येष्ठं क्षेपोत्रापि च क्षेपघातः ।। Bhāskara II, Bījaganita, VargaPrakṛti, 2-4

PRINCIPLE OF COMPOSITION 249 Principle of Composition The above results have been technically known amongst Indian algebraists as Bhāvanā (demonstrated or proved, hence theorem or lemma). The word bhāvanā also means "composition or combination" in algebra. Bhāvanā may be of two types : Samāsa Bhāvanā (or addition Lemma, or additive composition) and Antara Bhāvanā (or subtraction Lemma or subtractive composition). Whenever, again, the bhāvanā is made with two equal sets of roots and interpolators, it is technically named as Tulya Bhāvanā (or composition of equals), and when with two unequal sets of values then it is known as Atulya Bhāvanā (or composition of unequals). Proof of Brahmagupta's Lemmas It is significant to be indicated that Brahmagupta's Lemmas were rediscovered by Euler in 1764 and by Lagrange in 1768, and a considerable importance was attached to them. Kṛṣṇa, (1580 A.D.) the commentator on the Bījagaṇita of Bhāskara II gives the following proof of Brahmagupta's Lemmas : Let (α,β) and (α',β') be the two solutions of the equation nx² + k = y². we have Nα² + k = β² Nα'² + k' = β'² Multiplying the first equation by β'², we get Nα²β'² + kβ'² = β²β'² Now, substituting the value of factor β²' of the interpolator from the second equation, we get Nα² β'² + k (Nα'² + k') = β²β'² or N(α²β'² + Nkα'² + kk' = β²β'² Again, substituting the value of k from the first equation in the second term of the left-hand side expression, we have Nα²β'² + Nα'²(β² - Nα²) + kk' = β²β'² or N(α²β'² + α'²β²) + kk' = β²β'² + N²α²α'² Adding ±2Nαβα'β' to both sides, we get N(αβ' ± α'β)² + kk' = (ββ' ± Nαα')²

250 BRAHMAGUPTA AS AN ALGEBRAIST Brahmagupta's Corollary also follows at once from the above by putting α'=α, β'=β and k'=k. N (2αβ)²+k²=(β²±Nα²)² Thus the roots are x=2αβ and y=β²±Nα² which is the Corollary. It would be seen that modern historians of mathematics are incorrect when they say that Fermat (1657) was the first to state that the equation Nx²+1=y², where N is a non-square integer has an unlimited number of solutions in integers. For this assertion, history takes us to the early Seventh Century A.D. when Brahmagupta wrote his classical treatise, the Brāhmasphu- tasiddhānta, and gave the well known two Lemmas and the Corollary to the first Lemma. Second Lemma of Brahmagupta In the Brāhmasphuṭa siddhānta, we find another important Lemma by Brahmagupta stated as follows : On dividing the two roots (of a square- Nature) by the square-root of its additive or subtractive, the roots for interpolator unity (will be found).¹ This Lemma when expressed in the modern language of algebra would mean that if x=α, y=β be a solution of the equation. Nx²+k²=y² then x=α/k, y=β/k is a solution of the equation Nx²+1=y². This rule, at another place, has been re-enunciated as follows : If the interpolator is that divided by a square then the roots will be those multiplied by its square- root.²

1. प्रक्षेपशोधक हृते मूले प्रक्षेपके रूपे । —BrSpSi. XVIII. 65
2. वर्गाच्छिन्ने क्षेपे तत्पदगुणिते तदा मूले । —BrSpSi. XVIII. 70

SECOND LEMMA OF BRAHMAGUPTA 251 This rule may be expressed in terms of symbols as follows. Suppose the Varga-prakṛti (Square-nature) to be Nx² ± p²d = y², so that its interpolator (kṣepa) p²d is exactly divisible by the square p². Then, putting therein u = x/p, v = y/p, we derive the equation Nu² ± d = v² whose interpolator is equal to that of the original Square-nature divided by p². It is clear that the roots of the original equation are p times those of the derived equation. Rational Solution Indian algebraists have usually suggested the following method to obtain a first solution of Nx² + 1 = y² : Take an arbitrary small rational number, α, such that its square multiplied by the guṇaka N and increased or diminished by a suitably chosen rational number k will be an exact square. In other words, we shall have to obtain empirically a rela- tion of the form Nα² ± k = β² where α, k, and β are rational numbers. Let us call this relation as the Auxiliary Equation. Then by Brahmagupta's Coro- llary, we get from it the relation N(2αβ)² + k² = (β² + Nα²)², or N(2α β / k)² + 1 = ((β² + Nα²) / k)² Hence, one rational solution of the equation Nx² + 1 = y² is given by x = 2αβ / k , y = (β² + Nα²) / k Work on the rational solution of the Square-nature has been also done by Śrīpati. In fact, his solution, given in 1039 A.D. is of historical significance. He derives the rational solution without the aid of the "auxiliary equation." He gives the follo- wing rule :

252 BRAHMAGUPTA AS AN ALGEBRAIST Unity is the lesser root. Its square multiplied by the prakṛti is increased or decreased by the prakṛti com- bined with an (optional) number whose square-root will be the greater root. From them will be obtained two roots by the Principle of Composition¹ Thus if m² be the rational number optionally chosen, one shall have the identity : N.1²+(m²—N)=m², or N.1²—(N—m²)=m² Then by applying Brahmagupta's Corollary we get N(2m)²+(m² ∼ N)²=(m²+N)² ∴ N (2m / (m² ∼ N))² + 1 = ((m² + N) / (m² ∼ N))² Hence x = 2m / (m² ∼ N), y = (m² + N) / (m² ∼ N) where m is any rational number, is a solution of the equation Nx² + 1 = y². This rational solution of the varga-prakṛti which was used by Śrīpati in 1039 A.D. was rediscovered in Europe by Broun- cker in 1657. We shall close this discussion by taking an illustration from Bhāskara II : Problem : Tell me, O mathematician, what is that square which multiplied by 8 becomes, together with unity, a square; and what square multiplied by 11 and increased by unity, becomes a square. This means that we have to solve the equations : 8x² + 1 = y² ......(i) 11x² + 1 = y² ......(ii) In the second example, let us assume 1 as the lesser root. Following the method of Śrīpati, let us multiply its square by the prakṛti (here in eq. ii, prakṛti is 11), then let us subtract 2 (an optional number) and then extracting the square-roots we

1. Śrīpati, Siddhānta-śekhara XIV. 33

RATIONAL SOLUTION 253 get the greater root as 3. Hence the statement for the com- position is m=11 l=1 g=3 i=-2 l=1 g=3 i=-2 Here m=multiplier (guṇaka or prakṛti), l=lesser root (kaniṣṭha-mūla), g=greater root (jyeṣṭha-mūla) and i=interpola- tor (kṣepa). Here we have set down successively the lesser root, greater root and interpolator, and below them again set down the same (See Brahmagupta's Lemmas described by Bhāskara II). Now proceeding as before we obtain the roots for the additive 4 : l=6, g=20, (for) i=4. Then by the rule : "If the interpolator (of a varga-prakṛti or Square-nature) divided by the square of an optional number be the interpolator (of another Square-nature), then the two roots (of the former) divided by that optional number will be the roots (of the other). Or, if the interpolator be multiplied, their roots should be multiplied."¹ are found the roots for the additive unity l=3, g=10 (for) i=1. Whence by the Principle of Composition of Equals, we get the lesser and greater roots : l=60, g=199 (for) i= 1. In this way an infinite number of roots can be deduced. Alternative method:—Bhāskara II has given another method for finding the two roots for the additive unity : Or divide twice an optional number by the difference between the square of that optional number and the prakṛti. This (quotient) will be the lesser root (of a Square-nature) when unity is the additive. From that (follows) the greater root.²

1. इष्टवर्गहृतः क्षेपः क्षेपः स्यादिष्टभाजिते । मूले ते स्तोऽथवा क्षेपः क्षुण्णः क्षुण्णे तदा पदे ॥ Bījagaṇita II. 5.
2. Siddhānta-śekhara, XIV. 32.

254 BRAHMAGUPTA AS AN ALGEBRAIST Let us solve the first example 8x²＋1＝y². We assume the optional number to be 3. Its square is 9; the prakṛti of multiplier is 8, their difference is 9–8＝1. Dividing by this twice the optional number (2×3, i.e. 6), namely 6, we get the lesser root for the addi- tive unity as 6. Whence proceeding as before, we get the greater to be 17. Thus here x＝6 and y＝17. Let us use this method for the equation 11x²＋1＝y². Let the optional number be 3. Its square is 9: multiplier or prakṛti is 11; the difference is 11–9＝2; dividing by this twice the optional number (2×3), namely 6, we get 6/2＝3, which is the lesser root. Consequently the greater root would be 10. Thus for this equation x＝3 and y＝10. Solution in Positive Integers The Indian algebraists usually aimed at obtaining solutions of the varga-prakṛti or Square-nature in positive integers or abhinna. The tentative methods of Brahmagupta and Śrīpati always did not furnish solutions in positive integers. These auth- ors, however, discovered that if the interpolator of auxiliary equa- tion in the tentative method be ±1, ±2 or ±4, an integral solu- tion of the equation Nx²＋1＝y² can always be found. Thus Śrīpati says : If 1, 2 or 4 be the additive or subtractive (of the auxi- liary equation), the lesser and greater roots will be integral (abhinna)¹. (i) If k＝±1, then the auxiliary equation will be Nα²±1＝β where α and β are intergers. Then by Brahmagupta' Corollary we get x＝2αβ and y＝β²＋Nα² as the required first solution in positive integers of the equation Nx²＋1＝y²

1. इष्टवर्गं प्रकृत्योर्यद्विवरं तेन वा भजेत् । द्विघ्नमिष्टं कनिष्ठं तत् पदं स्यादेक संयुतौ । ततो ज्येष्ठमिहानन्त्यं भावनाभिस्तथेष्टतः ।। Bījagaṇita, Varga-Prakṛti, 5-6

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SOLUTION IN POSITIVE INTEGERS 255 (ii) Let k=±2; then the auxiliary equation is Nα²±2=β² By Brahmagupta's Corollary, we have N(2αβ)²+4=(β²+Nα²)² or N (αβ)²+1=( (β²+Nα²)/2 )² Hence the required first solution is x=αβ, y=½(β²+Nα²) Since Na²=β² ∓ 2, we have ½ (β²+Nα²)=β² ∓ 1=a whole number. (iii) Now suppose k=+4: so that Nα²+4=β² With an auxiliary equation like this, the first integral solution of the equation Nx²+1=y² is x=½αβ y=½(β²-2); if α is even; or x=½

256 BRAHMAGUPTA AS AN ALGEBRAIST Substituting the value of N in the right-hand side expres- sion from (i), we have N.( αβ / 2 )² + 1 = ( (β²-2) / 2 )² (iii) Composing (ii) and (iii), N { α/2 (β²-1) }² + 1 = { β/2 (β²-3) }² Hence x= ½ αβ, y= ½ (β²--2); and x= ½α(β²-1), y= ½β(β²-3); are solutions of Nx²+1=y². If β be even, the first values of (x,y) are integral. If β be odd, the second values are integral. (iv) Finally, suppose k=-4; the auxiliary equation is Nα²-4 = β² Then the required first solution in positive integers of Nx²+1=y² is x= ½αβ(β²+3) (β²+1) y=(β²+2) { ½(β²+3) (β²+1)-1 }. Brahmagupta says : In the case of 4 as subtractive, the square of the second is increased by three and by unity; half the product of these sums and that as diminished by unity (are obtained). The latter multiplied by the first sum less unity is the (required) second root; the former multi- plied by the product of the (old) roots will be the first root corresponding to the (new) second root.¹ The rationale of this solution, as given by Datta and Singh is as follows : Nα²-4=β² (i) N(α/2)²-1=(β/2)² Hence by Brahmagupta's Corollary, we get N ( αβ / 2 )² + 1 = ( β²/4 + N α²/4 )²

1. चतुरूनेऽन्यपद कृती त्र्येकयुते वधदलं पृथग्व्येकम् । व्येकाद्वाहतमन्त्यं पदवध गुणमाद्यमान्त्यपदम् ॥ BrSpSi. XVIII. 68

CAKRAVĀLA OR CYCLIC METHOD 257 = {½(β² + 2)}² (ii) Again applying the Corollary, we get N {½αβ(β² + 2)}² + 1 = {½(β⁴ + 4β² + 2)}² (iii) Now by the Lemma we obtain from (ii) and (iii) N {½αβ(β² + 3) (β² + 1)}² + 1 = [(β² + 2){½(β² + 3) (β² + 1) − 1}]² Hence x = ½αβ(β² + 3) (β² + 1), y = (β² + 2){½(β² + 3) (β² + 1) − 1} is a solution of Nx² + 1 = y² This can be proved without difficulty that these values of x and y are integral. Since if β is even, β² + 2 is also even. And hence the above values of x and y are integral. On the other hand, if β is odd, β² is also odd; under these conditions β² + 1 and β² + 3 are even. In this also, therefore, the above values must be integral. Putting p = αβ, q = β² + 2, we can write the above solution in the form x = ½p (q² − 1). y = ½q (q² − 3). This was the form in which the solution was found by Euler. Cakravāla or Cyclic Method We have shown in the preceding articles that the most fundamental step in Brahmagupta's method for the general solution in positive integers of the equation Nx² + 1 = y² where N is a non-square integer, is to form an auxiliary equation of the kind Na² + k = b² where a and b are positive integers and k = ±1, ±2 or ±4. From this auxiliary equation, by the Principle of Composition, applied repeatedly whenever necessary, one can derive, as we have alrea- dy shown above, one positive integral solution of the original Varga-prakṛti or Square-Nature. And thence again, by means of the same principle, an infinite number of other solutions in integers can be obtained. How to form an auxiliary equation of