Ramanujan's continued fractions of order sixteen

TheoremWeighted Lambert series for the continued fractions of order sixteen#●ready to prove

Write \((a; q)_\infty = \prod_{n \ge 0} (1 - a q^n)\), \((a_1, \dots, a_k; q)_\infty = (a_1; q)_\infty \cdots (a_k; q)_\infty\) and \(f_k = (q^k; q^k)_\infty\). For every complex number \(q\) with \(|q| < 1\),

\[\sum_{n \ge 1} n \frac{q^n - q^{7n} - q^{9n} + q^{15n}}{1 - q^{16n}} = \frac{q f_{16}^2 f_8^2 (q^6, q^{10}; q^{16})_\infty}{(q, q^7, q^9, q^{15}; q^{16})_\infty^2}\]

and

\[\sum_{n \ge 1} n \frac{q^{3n} - q^{5n} - q^{11n} + q^{13n}}{1 - q^{16n}} = \frac{q^3 f_{16}^2 f_8^2 (q^2, q^{14}; q^{16})_\infty} {(q^3, q^5, q^{11}, q^{13}; q^{16})_\infty^2}.\]
MetaMathlibExt.lambert_order_sixteen_weight_n · 15 lines
public theorem_wanted lambert_order_sixteen_weight_n (q : ℂ) (hq : ‖q‖ < 1) :
    (∑' n : ℕ, ((n : ℂ) + 1) *
        (q ^ (n + 1) - q ^ (7 * (n + 1)) - q ^ (9 * (n + 1)) + q ^ (15 * (n + 1))) /
          (1 - q ^ (16 * (n + 1)))) =
      q * qPochhammerInfRaw (q ^ 16) (q ^ 16) ^ 2 * qPochhammerInfRaw (q ^ 8) (q ^ 8) ^ 2 *
          (qPochhammerInfRaw (q ^ 6) (q ^ 16) * qPochhammerInfRaw (q ^ 10) (q ^ 16)) /
        (qPochhammerInfRaw q (q ^ 16) * qPochhammerInfRaw (q ^ 7) (q ^ 16) *
            qPochhammerInfRaw (q ^ 9) (q ^ 16) * qPochhammerInfRaw (q ^ 15) (q ^ 16)) ^ 2 ∧
    (∑' n : ℕ, ((n : ℂ) + 1) *
        (q ^ (3 * (n + 1)) - q ^ (5 * (n + 1)) - q ^ (11 * (n + 1)) + q ^ (13 * (n + 1))) /
          (1 - q ^ (16 * (n + 1)))) =
      q ^ 3 * qPochhammerInfRaw (q ^ 16) (q ^ 16) ^ 2 * qPochhammerInfRaw (q ^ 8) (q ^ 8) ^ 2 *
          (qPochhammerInfRaw (q ^ 2) (q ^ 16) * qPochhammerInfRaw (q ^ 14) (q ^ 16)) /
        (qPochhammerInfRaw (q ^ 3) (q ^ 16) * qPochhammerInfRaw (q ^ 5) (q ^ 16) *
            qPochhammerInfRaw (q ^ 11) (q ^ 16) * qPochhammerInfRaw (q ^ 13) (q ^ 16)) ^ 2
Sources
  • A. Vanitha, Nasser Abdo Saeed Bulkhali, and M. P. Chaudhary, On a Ramanujan’s Continued Fraction of Order Sixteen and New Eisenstein Series Identities, INTEGERS 25 (2025), Article A16, Theorem 3, Equations (62)-(63).
Uses 1

TheoremLambert series of order sixteen twisted by the Legendre symbol modulo \(3\)#●ready to prove

Write \((a; q)_\infty = \prod_{n \ge 0} (1 - a q^n)\), \((a_1, \dots, a_k; q)_\infty = (a_1; q)_\infty \cdots (a_k; q)_\infty\) and \(f_k = (q^k; q^k)_\infty\), and let \(\left(\frac{n}{3}\right)\) be the Legendre symbol modulo \(3\). For every complex number \(q\) with \(|q| < 1\),

\[\sum_{n \ge 1} \left(\frac{n}{3}\right) \frac{q^n - q^{7n} - q^{9n} + q^{15n}}{1 - q^{16n}} = \frac{q f_8^2 f_{48} (q^6, q^{10}; q^{16})_\infty (q, q^7, q^9, q^{15}; q^{16})_\infty} {f_{16} (q^3, q^{21}, q^{27}, q^{45}; q^{48})_\infty}\]

and

\[\sum_{n \ge 1} \left(\frac{n}{3}\right) \frac{q^{3n} - q^{5n} - q^{11n} + q^{13n}}{1 - q^{16n}} = \frac{q^3 f_8^2 f_{48} (q^2, q^{14}; q^{16})_\infty (q^3, q^5, q^{11}, q^{13}; q^{16})_\infty} {f_{16} (q^9, q^{15}, q^{33}, q^{39}; q^{48})_\infty}.\]
MetaMathlibExt.lambert_order_sixteen_legendre_three · 21 lines
public theorem_wanted lambert_order_sixteen_legendre_three (q : ℂ) (hq : ‖q‖ < 1) :
    (∑' n : ℕ, (jacobiSym ((n : ℤ) + 1) 3 : ℂ) *
        (q ^ (n + 1) - q ^ (7 * (n + 1)) - q ^ (9 * (n + 1)) + q ^ (15 * (n + 1))) /
          (1 - q ^ (16 * (n + 1)))) =
      q * qPochhammerInfRaw (q ^ 8) (q ^ 8) ^ 2 * qPochhammerInfRaw (q ^ 48) (q ^ 48) *
          (qPochhammerInfRaw (q ^ 6) (q ^ 16) * qPochhammerInfRaw (q ^ 10) (q ^ 16)) *
          (qPochhammerInfRaw q (q ^ 16) * qPochhammerInfRaw (q ^ 7) (q ^ 16) *
            qPochhammerInfRaw (q ^ 9) (q ^ 16) * qPochhammerInfRaw (q ^ 15) (q ^ 16)) /
        (qPochhammerInfRaw (q ^ 16) (q ^ 16) *
          (qPochhammerInfRaw (q ^ 3) (q ^ 48) * qPochhammerInfRaw (q ^ 21) (q ^ 48) *
            qPochhammerInfRaw (q ^ 27) (q ^ 48) * qPochhammerInfRaw (q ^ 45) (q ^ 48))) ∧
    (∑' n : ℕ, (jacobiSym ((n : ℤ) + 1) 3 : ℂ) *
        (q ^ (3 * (n + 1)) - q ^ (5 * (n + 1)) - q ^ (11 * (n + 1)) + q ^ (13 * (n + 1))) /
          (1 - q ^ (16 * (n + 1)))) =
      q ^ 3 * qPochhammerInfRaw (q ^ 8) (q ^ 8) ^ 2 * qPochhammerInfRaw (q ^ 48) (q ^ 48) *
          (qPochhammerInfRaw (q ^ 2) (q ^ 16) * qPochhammerInfRaw (q ^ 14) (q ^ 16)) *
          (qPochhammerInfRaw (q ^ 3) (q ^ 16) * qPochhammerInfRaw (q ^ 5) (q ^ 16) *
            qPochhammerInfRaw (q ^ 11) (q ^ 16) * qPochhammerInfRaw (q ^ 13) (q ^ 16)) /
        (qPochhammerInfRaw (q ^ 16) (q ^ 16) *
          (qPochhammerInfRaw (q ^ 9) (q ^ 48) * qPochhammerInfRaw (q ^ 15) (q ^ 48) *
            qPochhammerInfRaw (q ^ 33) (q ^ 48) * qPochhammerInfRaw (q ^ 39) (q ^ 48)))
Sources
  • A. Vanitha, Nasser Abdo Saeed Bulkhali, and M. P. Chaudhary, On a Ramanujan’s Continued Fraction of Order Sixteen and New Eisenstein Series Identities, INTEGERS 25 (2025), Article A16, Theorem 4, Equations (64)-(65).
Uses 1

TheoremProduct forms of Ramanujan's theta functions \(\psi\), \(f\) and \(\chi\)#●ready to prove

Write \((a; q)_\infty = \prod_{n \ge 0} (1 - a q^n)\) and \(f_k = (q^k; q^k)_\infty\), and let \(f(a, b) = \sum_{n \in \mathbb{Z}} a^{n(n+1)/2} b^{n(n-1)/2}\) be Ramanujan's general theta function. For every complex number \(q\) with \(|q| < 1\), \(f(q, q^3) = f_2^2 / f_1\), \(f(q, -q^2) = f_2^3 / (f_1 f_4)\), \((-q; q^2)_\infty = f_2^2 / (f_1 f_4)\) and \((q; q^2)_\infty = f_1 / f_2\).

In Ramanujan's notation (Berndt, Part III, Chapter 16), with \(\psi(q) = f(q, q^3)\), \(f(q) = f(q, -q^2)\) and \(\chi(q) = (-q; q^2)_\infty\), these read \(\psi(q) = f_2^2 / f_1\), \(f(q) = f_2^3 / (f_1 f_4)\), \(\chi(q) = f_2^2 / (f_1 f_4)\) and \(\chi(-q) = f_1 / f_2\).

public theorem_wanted ramanujan_psi_f_chi_eq_qPochhammer (q : ℂ) (hq : ‖q‖ < 1) :
    ramanujanThetaRaw q (q ^ 3) =
        qPochhammerInfRaw (q ^ 2) (q ^ 2) ^ 2 / qPochhammerInfRaw q q ∧
      ramanujanThetaRaw q (-q ^ 2) =
        qPochhammerInfRaw (q ^ 2) (q ^ 2) ^ 3 /
          (qPochhammerInfRaw q q * qPochhammerInfRaw (q ^ 4) (q ^ 4)) ∧
      qPochhammerInfRaw (-q) (q ^ 2) =
        qPochhammerInfRaw (q ^ 2) (q ^ 2) ^ 2 /
          (qPochhammerInfRaw q q * qPochhammerInfRaw (q ^ 4) (q ^ 4)) ∧
      qPochhammerInfRaw q (q ^ 2) = qPochhammerInfRaw q q / qPochhammerInfRaw (q ^ 2) (q ^ 2)
Sources
  • A. Vanitha, Nasser Abdo Saeed Bulkhali, and M. P. Chaudhary, On a Ramanujan’s Continued Fraction of Order Sixteen and New Eisenstein Series Identities, INTEGERS 25 (2025), Article A16, Lemma 1.
Uses 2

TheoremRamanujan's Entry 30 (i)–(iii) for the general theta function#●ready to prove

Let \(f(a, b) = \sum_{n \in \mathbb{Z}} a^{n(n+1)/2} b^{n(n-1)/2}\) be Ramanujan's general theta function and \(\psi(q) = f(q, q^3)\). For all complex numbers \(a\) and \(b\) with \(|ab| < 1\), \(f(a, ab^2) f(b, a^2 b) = f(a, b) \psi(ab)\) and \(f(a, b) + f(-a, -b) = 2 f(a^3 b, a b^3)\), and if moreover \(a \ne 0\), then \(f(a, b) - f(-a, -b) = 2a f(b/a, a^5 b^3)\).

public theorem_wanted ramanujanTheta_entry30 (a b : ℂ) (hab : ‖a * b‖ < 1) :
    ramanujanThetaRaw a (a * b ^ 2) * ramanujanThetaRaw b (a ^ 2 * b) =
        ramanujanThetaRaw a b * ramanujanThetaRaw (a * b) ((a * b) ^ 3) ∧
      ramanujanThetaRaw a b + ramanujanThetaRaw (-a) (-b) =
        2 * ramanujanThetaRaw (a ^ 3 * b) (a * b ^ 3) ∧
      (a ≠ 0 → ramanujanThetaRaw a b - ramanujanThetaRaw (-a) (-b) =
        2 * a * ramanujanThetaRaw (b / a) (a ^ 5 * b ^ 3))
Sources
  • A. Vanitha, Nasser Abdo Saeed Bulkhali, and M. P. Chaudhary, On a Ramanujan’s Continued Fraction of Order Sixteen and New Eisenstein Series Identities, INTEGERS 25 (2025), Article A16, Lemma 1 record, quoting Berndt, Part III, Entry 30 (i)-(iii).
Uses 1

TheoremOdd Lambert series for Ramanujan's continued fractions of order sixteen#●ready to prove

Write \((a; q)_\infty = \prod_{n \ge 0} (1 - a q^n)\) and \((a, b; q)_\infty = (a; q)_\infty (b; q)_\infty\). For a complex number \(q\) with \(|q| < 1\), put \(S(a, b) = \sum_{k \ge 0} (q^{a(2k+1)} + q^{b(2k+1)}) / (1 - q^{16(2k+1)})\), a sum over the odd multiples \(n = 2k + 1\), and \(E = q^4 (q^{32}; q^{32})_\infty^4 / (q^{16}; q^{16})_\infty^2\), \(I_1 = q (q^6, q^{26}; q^{32})_\infty / (q^{10}, q^{22}; q^{32})_\infty\) and \(I_2 = q^3 (q^2, q^{30}; q^{32})_\infty / (q^{14}, q^{18}; q^{32})_\infty\). Then \(S(3, 5) - S(11, 13) = E (I_1 + 1/I_1)\) and \(S(1, 7) - S(9, 15) = E (I_2 + 1/I_2)\). At \(q = 0\), where \(I_1 = I_2 = 0\), the quotients \(1/I_1\) and \(1/I_2\) are read as \(0\) and both sides of each identity are \(0\).

Here \(I_1\) and \(I_2\) are \(I_1(q^2)\) and \(I_2(q^2)\) for Ramanujan's continued fractions of order sixteen \(I_1(q) = q^{1/2} (q^3, q^{13}; q^{16})_\infty / (q^5, q^{11}; q^{16})_\infty\) and \(I_2(q) = q^{3/2} (q, q^{15}; q^{16})_\infty / (q^7, q^9; q^{16})_\infty\), and \(E\) is the eta quotient \(\eta^4(32\tau) / \eta^2(16\tau)\) for \(q = e^{2 \pi i \tau}\), where \(\eta(\tau) = q^{1/24} (q; q)_\infty\).

public theorem_wanted lambert_odd_order_sixteen_eq_continuedFraction (q : ℂ) (hq : ‖q‖ < 1) :
    let E : ℂ := q ^ 4 * qPochhammerInfRaw (q ^ 32) (q ^ 32) ^ 4 /
      qPochhammerInfRaw (q ^ 16) (q ^ 16) ^ 2
    let I₁ : ℂ := q * (qPochhammerInfRaw (q ^ 6) (q ^ 32) * qPochhammerInfRaw (q ^ 26) (q ^ 32)) /
      (qPochhammerInfRaw (q ^ 10) (q ^ 32) * qPochhammerInfRaw (q ^ 22) (q ^ 32))
    let I₂ : ℂ := q ^ 3 *
      (qPochhammerInfRaw (q ^ 2) (q ^ 32) * qPochhammerInfRaw (q ^ 30) (q ^ 32)) /
      (qPochhammerInfRaw (q ^ 14) (q ^ 32) * qPochhammerInfRaw (q ^ 18) (q ^ 32))
    (∑' k : ℕ, (q ^ (3 * (2 * k + 1)) + q ^ (5 * (2 * k + 1))) / (1 - q ^ (16 * (2 * k + 1))) -
        ∑' k : ℕ, (q ^ (11 * (2 * k + 1)) + q ^ (13 * (2 * k + 1))) /
          (1 - q ^ (16 * (2 * k + 1)))) = E * (I₁ + 1 / I₁) ∧
    (∑' k : ℕ, (q ^ (2 * k + 1) + q ^ (7 * (2 * k + 1))) / (1 - q ^ (16 * (2 * k + 1))) -
        ∑' k : ℕ, (q ^ (9 * (2 * k + 1)) + q ^ (15 * (2 * k + 1))) /
          (1 - q ^ (16 * (2 * k + 1)))) = E * (I₂ + 1 / I₂)
Sources
  • A. Vanitha, Nasser Abdo Saeed Bulkhali, and M. P. Chaudhary, On a Ramanujan’s Continued Fraction of Order Sixteen and New Eisenstein Series Identities, INTEGERS 25 (2025), Article A16, Theorem 2, Equations (54)-(55).
Uses 1

DefinitionRaw Jacobi theta function \(\theta_1\)#✓fully proved

For complex numbers \(z\) and \(\tau\), the raw Jacobi theta function is

\[\theta_1(z \mid \tau) = 2 \sum_{n=0}^{\infty} (-1)^n e^{\pi i \tau (2n+1)^2/4} \sin((2n+1) z)\]

when this series is summable, and \(0\) otherwise. With \(q = e^{2 \pi i \tau}\), the factor \(e^{\pi i \tau (2n+1)^2/4}\) is \(q^{(2n+1)^2/8}\).

The value \(0\) for a non-summable series is a convention; it occurs, for example, at \(\tau = 0\) and \(z = \pi/2\). The intended object is the Jacobi theta function on the upper half-plane \(\operatorname{Im} \tau > 0\), where the series converges for every \(z\).

noncomputable def jacobiThetaOneSineRaw (z τ : ℂ) : ℂ :=
  2 * ∑' n : ℕ, (-1) ^ n * Complex.exp (Real.pi * Complex.I * τ * (2 * n + 1) ^ 2 / 4) *
    Complex.sin ((2 * n + 1) * z)
Sources
  • A. Vanitha, Nasser Abdo Saeed Bulkhali, and M. P. Chaudhary, On a Ramanujan’s Continued Fraction of Order Sixteen and New Eisenstein Series Identities, INTEGERS 25 (2025), Article A16, Section 3, Equation (11).
Used by 1

DefinitionJacobi theta function \(\theta_1\)#✓fully proved

For complex numbers \(z\) and \(\tau\) with \(\operatorname{Im} \tau > 0\), the Jacobi theta function is

\[\theta_1(z \mid \tau) = 2 \sum_{n=0}^{\infty} (-1)^n e^{\pi i \tau (2n+1)^2/4} \sin((2n+1) z).\]

With \(q = e^{2 \pi i \tau}\), the factor \(e^{\pi i \tau (2n+1)^2/4}\) is \(q^{(2n+1)^2/8}\).

On the upper half-plane \(\operatorname{Im} \tau > 0\) the series converges for every \(z\).

noncomputable def jacobiThetaOneSine (z τ : ℂ) (_hτ : 0 < τ.im) : ℂ :=
  jacobiThetaOneSineRaw z τ
Sources
  • A. Vanitha, Nasser Abdo Saeed Bulkhali, and M. P. Chaudhary, On a Ramanujan’s Continued Fraction of Order Sixteen and New Eisenstein Series Identities, INTEGERS 25 (2025), Article A16, Section 3, Equation (11).
Uses 1 · Used by 3

TheoremThe Jacobi theta function \(\theta_1\) vanishes at \(z = 0\)#✓fully proved

For every complex number \(\tau\) with \(\operatorname{Im} \tau > 0\), the Jacobi theta function satisfies \(\theta_1(0 \mid \tau) = 0\).

theorem jacobiThetaOneSine_zero (τ : ℂ) (hτ : 0 < τ.im) : jacobiThetaOneSine 0 τ hτ = 0
Sources
  • A. Vanitha, Nasser Abdo Saeed Bulkhali, and M. P. Chaudhary, On a Ramanujan’s Continued Fraction of Order Sixteen and New Eisenstein Series Identities, INTEGERS 25 (2025), Article A16, Section 3, Equation (11).
Uses 1

TheoremOddness of the Jacobi theta function \(\theta_1\)#✓fully proved

For all complex numbers \(z\) and \(\tau\) with \(\operatorname{Im} \tau > 0\), the Jacobi theta function satisfies \(\theta_1(-z \mid \tau) = -\theta_1(z \mid \tau)\).

theorem jacobiThetaOneSine_neg (z τ : ℂ) (hτ : 0 < τ.im) :
    jacobiThetaOneSine (-z) τ hτ = -jacobiThetaOneSine z τ hτ
Sources
  • A. Vanitha, Nasser Abdo Saeed Bulkhali, and M. P. Chaudhary, On a Ramanujan’s Continued Fraction of Order Sixteen and New Eisenstein Series Identities, INTEGERS 25 (2025), Article A16, Section 3, Equation (11).
Uses 1

TheoremSine Lambert series of order sixteen as quotients of theta functions#●ready to prove

Let \(\tau\) and \(z\) be complex numbers with \(\operatorname{Im} \tau > 0\) and \(|\operatorname{Im} z| < \pi \operatorname{Im} \tau\), let \(q = e^{2 \pi i \tau}\), write \(\theta(w) = \theta_1(w \mid 16\tau)\) for the Jacobi theta function \(\theta_1\), and let \(\theta'\) be the derivative of \(\theta\). Then

\[\sum_{n \ge 1} \frac{q^n - q^{7n} - q^{9n} + q^{15n}}{1 - q^{16n}} \sin 2nz = \frac{-\theta'(0) \theta(8\pi\tau) \theta(6\pi\tau) \theta(2z)} {4 \theta(z + \pi\tau) \theta(z - \pi\tau) \theta(z + 7\pi\tau) \theta(z - 7\pi\tau)}\]

and

\[\sum_{n \ge 1} \frac{q^{3n} - q^{5n} - q^{11n} + q^{13n}}{1 - q^{16n}} \sin 2nz = \frac{-\theta'(0) \theta(2\pi\tau) \theta(8\pi\tau) \theta(2z)} {4 \theta(z + 3\pi\tau) \theta(z - 3\pi\tau) \theta(z + 5\pi\tau) \theta(z - 5\pi\tau)}.\]

Under these hypotheses the series converge and the denominators do not vanish.

public theorem_wanted lambert_order_sixteen_sine_eq_jacobiThetaOne (τ z : ℂ) (hτ : 0 < τ.im)
    (hz : |z.im| < Real.pi * τ.im) :
    let q : ℂ := Complex.exp (2 * Real.pi * Complex.I * τ)
    let θ : ℂ → ℂ := fun w => jacobiThetaOneSine w (16 * τ) (by simp [hτ])
    let πτ : ℂ := Real.pi * τ
    (∑' n : ℕ, (q ^ (n + 1) - q ^ (7 * (n + 1)) - q ^ (9 * (n + 1)) + q ^ (15 * (n + 1))) /
        (1 - q ^ (16 * (n + 1))) * Complex.sin (2 * ((n : ℂ) + 1) * z)) =
      -(deriv θ 0 * θ (8 * πτ) * θ (6 * πτ) * θ (2 * z)) /
        (4 * θ (z + πτ) * θ (z - πτ) * θ (z + 7 * πτ) * θ (z - 7 * πτ)) ∧
    (∑' n : ℕ, (q ^ (3 * (n + 1)) - q ^ (5 * (n + 1)) - q ^ (11 * (n + 1)) +
          q ^ (13 * (n + 1))) / (1 - q ^ (16 * (n + 1))) * Complex.sin (2 * ((n : ℂ) + 1) * z)) =
      -(deriv θ 0 * θ (2 * πτ) * θ (8 * πτ) * θ (2 * z)) /
        (4 * θ (z + 3 * πτ) * θ (z - 3 * πτ) * θ (z + 5 * πτ) * θ (z - 5 * πτ))
Sources
  • A. Vanitha, Nasser Abdo Saeed Bulkhali, and M. P. Chaudhary, On a Ramanujan’s Continued Fraction of Order Sixteen and New Eisenstein Series Identities, INTEGERS 25 (2025), Article A16, Lemma 2 of Section 5, Equations (59)-(60).
Uses 1
4 fully proved6 ready to prove

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