An Interesting q-continued Fractions of Ramanujan

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1 Palestine Journal of Mathematics Vol. 4(1 (015, Palestine Polytechnic University-PPU 015 An Interesting q-continued Fractions of Ramanujan S. N. Fathima, T. Kathiravan Yudhisthira Jamudulia Communicated by Zafar Ahsan MSC 010 Classifications: 11A55. Keywords phrases: Continued fractions, Modular Equations. The first-named author is thankful to UGC, New Delhi for awarding research project [No. F41-139/01/(SR], under which this work has been done. Abstract. In this paper, we establish an interesting q-identity an integral representation of a q-continued fraction of Ramanujan. We also compute explicit evaluation of this continued fraction derive its relation with Ramanujan Göllnitz -Gordon continued fraction. 1 Introduction Ramanujan a pioneer in the theory of continued fraction has recorded several in the process rediscovered few continued fractions found earlier by Gauss, Eisenstein Rogers in his notebook [10]. In fact Chapter 1 Chapter 16 of his Second Notebook [10] is devoted to continued fractions. Proofs of these continued fractions over years are given by several mathematician, we mention here specially G.E. Andrews[3], C. Adiga, S. Bhargava G.N. Watson [1] whose works have been compiled in [4] [5]. The celebrated Roger Ramanujan continued fraction is defined by R(q := 1 q q q 3, q < 1. On page 365 of his lost notebook [11], Ramanujan recorded five modular equations relating R(q with R( q, R(q, R(q 3 R(q 4 R(q 5. The well known Ramanujan s cubic continued fraction defined by J(q := q1/3 q + q q + q 4 q 3 + q 6, q < 1. is recorded on page 366 of his lost notebook [11]. Several new modular equation relating J(q with J( q, J(q J(q 3 are established by H.H. Chan [8]. Similarly the Ramanujan Göllnitz-Gordon continued fraction K(q defined by K(q := q1/ q + q q 3 + q 4 q 5 + q 6 q 7 +, q < 1, satisfies several beautiful modular relations. One may see traces of modular equation related to K(q on page 9 of Ramanujan s lost notebook [11]. Further works related to K(q in recent years have been done by various authors including Chan S.S Huang [9] K.R. Vasuki B.R. Srivatsa Kumar [1]. Motivated by these works in this paper we study the Ramanujan continued fraction M(q := q1/ 1 q + q(1 q (1 q( q + q(1 q 3 (1 q( q 4 + q(1 q 5 (1 q( q 6 +, q < 1 = q 1/ (q4 ; q 4 (q ; q 4. (1.1 In Chapter 16 Entry 1 of [5], Ramanujan has recorded the following continued fraction (a q 3 ; q 4 (b q 3 ; q 4 (a q; q 4 (b q; q 4 = 1 1 ab + (a bq(b aq (1 ab( q + (a bq 3 (b aq 3 (1 ab( q 4 +, ab < 1, q < 1. (1.

2 An Interesting q-continued Fractions of Ramanujan 199 Ra- In fact setting a = q 1/ b = q 1/ in (1., we obtain (1.1. In Section we obtain an interesting q-identity related to M(q using manujan s 1 ψ 1 summation formula [5, Ch. 16, Entry 17] n= Andrew s identity [4, p. 57], (a n (b n z n = (az (q/az (q (b/a (z (b/az (b (q/a, b/a < z < 1, (1.3 q kn 1 q = ln+k q ln +kn qln+k. (1.4 1 qln+k In Section 3 we obtain several relation of M(q with theta function ϕ(q, ψ(q χ(q. In Section 4 we obtain an integral representation of M(q. In Section 5 we derive a formula that help us to obtain relation among M(q 1/, M(q, M(q M(q 4. We establish explicit formulas for the evaluation of M(e πn in Section 6. We conclude this introduction with few customary definition we make use in the sequel. For a q complex number with q < 1 (a := (a; q = (a n := (a; q n = (1 aq n n 1 (1 aq k = (a (aq n, k=0 n : any integer. f(a, b := n= a n(n+1/ b n(n 1/ (1.5 = ( a; ab ( b; ab (ab; ab, ab < 1. (1.6 Identity (1.6 is the Jacobi s triple product identity in Ramanujan s notation 16, Entry 19]. It follows from (1.5 (1.6 that [5, Ch. 16, Entry ], [5, Ch. ϕ(q := f(q, q = n= q n = ( q; q (q; q, (1.7 ψ(q := f(q, q 3 = q n(n+1/ = (q ; q (q; q, (1.8 χ(q := ( q; q. (1.9 q-identity related to M(q Theorem.1 M(q = n(8n+4+1/ q8n+ q 1 q 8n+ (n+1(8n+4+1/ q8n+6 q 1 q 8n+6 (.1 Proof: Changing q to q 8, then setting a = q, b = q 10 z = q in 1 ψ 1 summation formula (1.3 we obtain (q 4 ; q 4 (q ; q 4 = q n 1 q 8n+ q 6n+4 1 q 8n+6 (.

3 00 S. N. Fathima, T. Kathiravan Yudhisthira Jamudulia employing Andrews identity (1.4 with k =, l = 8 k = 6, l = 8 in both the summations in right side of the identity (. respectively finally multiplying both sides of the resulting identity with q 1/ using product represtation of M(q (1.1, we complete the proof of Theorem.1. 3 Some Identities involving M(q We obtain relation of M(q in terms of theta function ϕ(q, ψ(q χ(q. Theorem 3.1 M(q = q 1/ ψ4 (q ϕ (q, (3.1 8M(q = ϕ (q ϕ ( q, (3. 16M (q = ϕ 4 (q ϕ 4 ( q, (3.3 M (q M(q M 1 (q + M(q M 1 (q M(q = ϕ (q, (3.4 4M(q = ϕ (q ϕ (q, (3.5 = qψ4 (q 1 qψ 4 (q, (3.6 8M(q = χ (q χ ( q φ ( q φ ( q. (3.7 Proof: Using [5, Ch. 16, Entry (ii] in (1.1, we obtain M(q = q 1/ ψ (q. (3.8 Employing [5, Ch. 16, Entry 5(iv] in (3.8, we obtain (3.1. From (3.1 we have M(q = q ψ4 (q ϕ (q. (3.9 Employing [5, Ch. 16, Entry 5(vii] [5, Ch. 16, Entry 5(vi] in (3.9, we obtain (3.. Identity (3.3 immediately follows from (3.8 [5, Ch. 16, Entry 5(vii]. Again from (3.1, we have M (q M(q = ψ8 (qϕ (q ψ 4 (q ϕ 4 (q, (3.10 employing [5, Ch. 16, Entry 5(iv] in the identity (3.10 we obtain (3.4. From (3. (3.3, we have 64M (q + 16M (q = 16ϕ (qm(q, (3.11 dividing the identity (3.11 throughout by 16M(q using (3.4 we obtain (3.5. From (3.1 we deduce that M 1 (q + M(q = ϕ4 (q + q ψ 8 (q q 1/ ϕ (qψ 4 (q, (3.1 M 1 (q M(q = ϕ4 (q q ψ 8 (q q 1/ ϕ (qψ 4 (q. (3.13 On dividing (3.1 by (3.13 using [5, Ch. 16, Entry 5(iv] in the resulting identity, we complete the proof of (3.6. From (1.7 (1.9 we have ϕ( q + χ(q χ( q ϕ( q = (q; q [ ( q; q ] f(q, q, f( q, q

4 An Interesting q-continued Fractions of Ramanujan 01 employing [5, Ch. 16, Entry 30(ii] in right h side of above identity we obtain ϕ( q + χ(q χ( q ϕ( q = (q 8 ; q 8 5 (q 3 ; q 3 (q 4 ; q 4 (q 16 ; q 16 (q 64 ; q 64 4 M(q 16 q 8. (3.14 Again from (1.7 (1.9 we have ϕ( q χ(q χ( q ϕ( q = (q; q [ 1 ( q; q ] f(q, q, f( q, q employing [5, Ch. 16, Entry 30(ii] [5, Ch. 16, Entry 18(ii] in right h side of above identity we obtain ϕ( q χ(q χ( q ϕ( q = 4q( q8 ; q 8 (q 64 ; q 64 3 q 8 ( q 16 ; q 3 M(q 16. (3.15 Multiplying (3.14 (3.15 we complete the proof of (3.7. Theorem 3.. Let u = M(q, v = M( q w = M(q, then u v = 8w Proof: On substituting (3.4 in (3.5, we obtain ϕ (q = 4M (q + M (q M(q Changing q to q in (3.16, we have ϕ ( q = 4M (q + M ( q M(q. (3.16. (3.17 Subtracting (3.17 from (3.16 using identity (3., we complete the proof of Theorem Integral Representation of M(q Theorem 4.1. For 0 < q < 1, ( 1 M(q = exp q + 4 [ ϕ 4 ( q 1 q 8 where ϕ(q ψ(q are as defined in (1.7 (1.8. Proof: Taking log on both sides of (3.1, we have + q ] ϕ (q dq, (4.1 ϕ(q log M(q = 1 log q + 4 log ψ(q log ϕ(q. (4. Employing [5, Ch. 16, Entry 3(ii] [5, Ch. 16, Entry 3(i] on right h side of (4., we obtain log M(q = 1 log q + 4 q n n( q n. (4.3 n=1 Differentiating (4.3 simplifying, we have d dq log M(q = 1 q + 4 [ q n=1 ( 1 n q n ( q n + n=1 ] q n 1 ( q n 1. (4.4 Using Jacobi s identity [5, Ch. 16, Identity 33.5, p. 54] [5, Ch. 16, Entry 3(i] integrating both sides finally exponentiating both sides of identity (4.4, we complete the proof of Theorem 4.1.

5 0 S. N. Fathima, T. Kathiravan Yudhisthira Jamudulia 5 Modular Equation of Degree n Relation Between M(q M(q n In the terminology of hypergeometric function, a modular equation of degree n is a relation between α β that is induced by where n F 1 (1/, 1/; 1; 1 α F 1 (1/, 1/; 1; α F 1 (a, b; c; x = (a k = k=0 = F 1 (1/, 1/; 1; 1 β F 1 (1/, 1/; 1; β (a k (b k (c k k! Γ(a + k Γ(a. x k,, Let Z 1 (r = F 1 (1/r, r 1/r; 1; α Z n (r = F 1 (1/r, r 1/r; 1; β, where n is the degree of the modular equation. The multiplier m(r is defined by the equation Theorem 5.1. If then m(r = Z 1(r Z n (r. ( q = exp α = 16 M 4 (q M 4 (q 1/ Proof: From (1.1 (1.7, we have M(qϕ (q = q 1/ (q4 ; q 4 (q ; q 4 π F 1 (1/, 1/; 1; 1 α F 1 (1/, 1/; 1; α ( q; q 4 (q 4 ; q 4 ( q ; q 4 (q ; q 4, (5.1 = M (q 1/. (5.3 Substituting (5.3 in (3.3, we obtain 16M (q = M 4 (q 1/ [ M (q 1 ϕ4 ( q ϕ 4 (q ]. (5.4 From a known identity [5, Ch. 16, p. 100, Entry 5] (5.1 it is implied that α = 1 ϕ4 ( q ϕ 4 (q. (5.5 Using (5.5 in (5.4, we complete the proof of (5.. Let α β be related by (5.1. If β has degree n over α then from Theorem 5.1, we obtain β = 16 M 4 (q n M 4 (q n/. (5.6 (5. Corollary 5.. Let u = M(q 1/, v = M(q, w = M(q x = M(q 4, then 16x 4 v + 3x 3 wv 4x 3 wu 4 + 4x w v + 8xw 3 v xw 3 u 4 + w 4 v = 0. (5.7 Proof: From [5, Entry 4(v, p. 16], we have ( 1 β 1/4 1 α = β 1/4. (5.8

6 An Interesting q-continued Fractions of Ramanujan 03 On using (5.6 with n = 4 (5. in (5.8, we obtain u 4 16v 4 ( w x u 4 =. (5.9 w + x Squaring both side of (5.9 then simplifying, we obtain ( Evaluations of M(q As an application of Theorem 5.1, we establish few explicit evaluation of M(q. Let q n = e πn let α n denote the corresponding value of α in (5.1. Then by Theorem 5.1, we have M(e πn = 1 α1/4 n (6.1 From [5, Ch. 17, p. 97], we have α 1 = 1, α = ( 1 α 4 = ( 1 4. Thus from (6.1, it immediately follows M(e π M(e π/ M(e π M(e π/ M(e π M(e π = ( 5/4 1, (6. = 1 1, (6.3 = 1. (6.4 Ramanujan has recorded several modular equation in his notebook [10, p ] [10, p ] which are very useful in the computation of class invariants the values of theta function. Ramanujan has also recorded several values of theta function ϕ(q ψ(q in his notebook. For example ϕ(e π = π 1/4 Γ(3/4, (6.5 ψ(e π = 5/8 e π/8 π 1/4 ϕ(e π ϕ(e 3π From (3.8 (6.6, we have M(e π/ = 5/4 Γ(3/4, (6.6 = (6.7 π Γ (3/4, (6.8 Using (6.8 (6., we obtain π M(e π = Γ (3/. (6.9 Setting (6.9 in (6.4, we obtain 1 M(e π = π Γ (3/. (6.10 J.M. Borwein P.B. Borwein [7] are the first to observe that class invariant could be used

7 04 S. N. Fathima, T. Kathiravan Yudhisthira Jamudulia to evaluated certain values of ϕ(e nπ. The Ramanujan Weber class invariants are defined by G n := 1/4 qn 1/4 ( q n ; qn g n := 1/4 q 1/4 n (q n ; q n, (6.11 where q n = e πn. Chan Huang has derived few explicit formulas for evaluating K(e πn/ in the terms of Ramanujan Weber class. Similar works are done by Adiga et., al. Analogoues to n these works we obtain explicit formulas to evaluate M(e π. Theorem 6.1. For Ramanujan Weber class invariant defined as in (6.11, let p = G 1 n p 1 = gn 1, then M(e πn M(e πn = 1 1 p (p p (p 1, (6.1 = 1 p 1 p 1. (6.13 Proof: From [9], we have Hence G n = [4α n (1 α n ] 1/4. α n = Using (6.14 in (6.1, we obtain (6.1. Also from [9], we have Hence 1 ( p(p p(p 1. (6.14 g 1 n = 1 αn α n. αn = (p 1 p 1. (6.15 Using (6.15 in (6.1, we complete the proof of (6.13. Example: Let n = 1. Since G 1 = 1, from Theorem 6.1 we have M(e π M(e π/ = ( 5/4 1. Let n =. Since g = 1, from Theorem 6.1 we have M(e π M(e π/ = 1 1. Remark: Using [10, p. 9] it is easily verified that M(q K(q are related by the equation M(q K(q + K(qM(q M(q = 0. References [1] C. Adiga, B. C. Berndt, S.Bhargava G. N. Watson, Chapter 16 of Ramanujan s Second Notebook: Theta-Functions q-series, Mem. Amer. Math. Soc., No. 315, 53(1985, 1-85, Amer. Math. Soc., Providence, [] C. Adiga, T. Kim, M. S. Mahadeva Naika H. S. Madhusudhan, On Ramanujan s Cubic Continued Fraction Explicit Evaluations of Theta Functions,Indian J. Pure App. Math., No. 55, 9(004,

8 An Interesting q-continued Fractions of Ramanujan 05 [3] G. E. Andrews, An introduction to Ramanujan s lost notebook, Amer. Math. Monthly, 86(1979, [4] G. E. Andrews, The Lost Notebook Part I, Springer Verlang, New York, 005. [5] B. C. Berndt, Ramanujan s Notebooks, Part III, Springer-Verlag, New York, [6] B. C. Berndt, Ramanujan s Notebooks, Part V, Springer-Verlag, New York, [7] J. M. Borwein P. B. Borwein, Pi the AGM, Wiley, New York, [8] H.H. Chan, On Ramanujan s Cubic Continued Fraction, Acta Arith. 73(1995, [9] H.H. Chan S.S. Huang, On the Ramanujan Göllnitz Gordon Continued Fraction, Ramanujan J., 1(1997, [10] S. Ramanujan, Notebooks, volumes, Tata Institute of Fundamental Research, Bombay, [11] S. Ramanujan, The Lost Notebook Other Unpublished Papers, Narosa, New Delhi, [1] K.R. Vasuki B.R. Srivatsa Kumar, Certain identities for the Ramanujan Gollnitz Gordon Continued fraction, J. of Comp. Applied Mathematics. 187(006, Author information S. N. Fathima, T. Kathiravan Yudhisthira Jamudulia, Department of Mathematics, Ramanujan School of Mathematics, Pondicherry University, Puducherry , INDIA. fathima.mat@pondiuni.edu.in; kkathiravan98@gmail.com; yudhis4581@yahoo.co.in Received: October 7, 013. Accepted: January 11, 014.