# acb_dirichlet.h – Dirichlet L-functions, Riemann zeta and related functions

This module allows working with values of Dirichlet characters, Dirichlet L-functions, and related functions. A Dirichlet L-function is the analytic continuation of an L-series

\[L(s,\chi) = \sum_{k=1}^\infty \frac{\chi(k)}{k^s}\]

where \(\chi(k)\) is a Dirichlet character. The trivial character \(\chi(k) = 1\) gives the Riemann zeta function. Working with Dirichlet characters is documented in dirichlet.h – Dirichlet characters.

The code in other modules for computing the Riemann zeta function, Hurwitz zeta function and polylogarithm will possibly be migrated to this module in the future.

## Roots of unity

type acb_dirichlet_roots_struct

type acb_dirichlet_roots_t

void acb_dirichlet_roots_init(acb_dirichlet_roots_t roots, ulong n, slong num, slong prec)

  Initializes roots with precomputed data for fast evaluation of roots of unity \(e^{2\pi i k/n}\) of a fixed order n. The precomputation is optimized for num evaluations.

  For very small num, only the single root \(e^{2\pi i/n}\) will be precomputed, which can then be raised to a power. For small prec and large n, this method might even skip precomputing this single root if it estimates that evaluating roots of unity from scratch will be faster than powering.

  If num is large enough, the whole set of roots in the first quadrant will be precomputed at once. However, this is automatically avoided for large n if too much memory would be used. For intermediate num, baby-step giant-step tables are computed.

void acb_dirichlet_roots_clear(acb_dirichlet_roots_t roots)

  Clears the structure.

void acb_dirichlet_root(acb_t res, const acb_dirichlet_roots_t roots, ulong k, slong prec)

  Computes \(e^{2\pi i k/n}\).

## Truncated L-series and power sums

void acb_dirichlet_powsum_term(acb_ptr res, arb_t log_prev, ulong *prev, const acb_t s, ulong k, int integer, int critical_line, slong len, slong prec)

  Sets res to \(k^{-(s+x)}\) as a power series in x truncated to length len. The flags integer and critical_line respectively specify optimizing for s being an integer or having real part 1/2.

  On input log_prev should contain the natural logarithm of the integer at prev. If prev is close to k, this can be used to speed up computations. If \(\log(k)\) is computed internally by this function, then log_prev is overwritten by this value, and the integer at prev is overwritten by k, allowing log_prev to be recycled for the next term when evaluating a power sum.

void acb_dirichlet_powsum_sieved(acb_ptr res, const acb_t s, ulong n, slong len, slong prec)

  Sets res to \(\sum_{k=1}^n k^{-(s+x)}\) as a power series in x truncated to length len. This function stores a table of powers that have already been calculated, computing \((ij)^r\) as \(i^r j^r\) whenever \(k = ij\) is composite. As a further optimization, it groups all even \(k\) and evaluates the sum as a polynomial in \(2^{-(s+x)}\). This scheme requires about \(n / \log n\) powers, \(n / 2\) multiplications, and temporary storage of \(n / 6\) power series. Due to the extra power series multiplications, it is only faster than the naive algorithm when len is small.

void acb_dirichlet_powsum_smooth(acb_ptr res, const acb_t s, ulong n, slong len, slong prec)

  Sets res to \(\sum_{k=1}^n k^{-(s+x)}\) as a power series in x truncated to length len. This function performs partial sieving by adding multiples of 5-smooth k into separate buckets. Asymptotically, this requires computing 4/15 of the powers, which is slower than sieved, but only requires logarithmic extra space. It is also faster for large len, since most power series multiplications are traded for additions. A slightly bigger gain for larger n could be achieved by using more small prime factors, at the expense of space.

## Riemann zeta function

void acb_dirichlet_zeta(acb_t res, const acb_t s, slong prec)

  Computes \(\zeta(s)\) using an automatic choice of algorithm.

void acb_dirichlet_zeta_jet(acb_t res, const acb_t s, int deflate, slong len, slong prec)

  Computes the first len terms of the Taylor series of the Riemann zeta function at s. If deflate is nonzero, computes the deflated function \(\zeta(s) - 1/(s-1)\) instead.

void acb_dirichlet_zeta_bound(mag_t res, const acb_t s)

  Computes an upper bound for \(|\zeta(s)|\) quickly. On the critical strip (and slightly outside of it), formula (43.3) in [Rad1973] is used. To the right, evaluating at the real part of s gives a trivial bound. To the left, the functional equation is used.

void acb_dirichlet_zeta_deriv_bound(mag_t der1, mag_t der2, const acb_t s)

  Sets der1 to a bound for \(|\zeta'(s)|\) and der2 to a bound for \(|\zeta''(s)|\). These bounds are mainly intended for use in the critical strip and will not be tight.

void acb_dirichlet_eta(acb_t res, const acb_t s, slong prec)

  Sets res to the Dirichlet eta function \(\eta(s) = \sum_{k=1}^{\infty} (-1)^{k+1} / k^s = (1-2^{1-s}) \zeta(s)\), also known as the alternating zeta function. Note that the alternating character \(\{1,-1\}\) is not itself a Dirichlet character.

void acb_dirichlet_xi(acb_t res, const acb_t s, slong prec)

  Sets res to the Riemann xi function \(\xi(s) = \frac{1}{2} s (s-1) \pi^{-s/2} \Gamma(\frac{1}{2} s) \zeta(s)\). The functional equation for xi is \(\xi(1-s) = \xi(s)\).

## Riemann-Siegel formula

The Riemann-Siegel (RS) formula is implemented closely following J. Arias de Reyna [Ari2011]. For \(s = \sigma + it\) with \(t > 0\), the expansion takes the form

\[\zeta(s) = \mathcal{R}(s) + X(s) \overline{\mathcal{R}}(1-s), \quad X(s) = \pi^{s-1/2} \frac{\Gamma((1-s)/2)}{\Gamma(s/2)}\]

where

\[\mathcal{R}(s) = \sum_{k=1}^N \frac{1}{k^s} + (-1)^{N-1} U a^{-\sigma} \left[ \sum_{k=0}^K \frac{C_k(p)}{a^k} + RS_K \right]\]
\[U = \exp\left(-i\left[ \frac{t}{2} \log\left(\frac{t}{2\pi}\right)-\frac{t}{2}-\frac{\pi}{8} \right]\right), \quad a = \sqrt{\frac{t}{2\pi}}, \quad N = \lfloor a \rfloor, \quad p = 1-2(a-N).\]

The coefficients \(C_k(p)\) in the asymptotic part of the expansion are expressed in terms of certain auxiliary coefficients \(d_j^{(k)}\) and \(F^{(j)}(p)\). Because of artificial discontinuities, s should be exact inside the evaluation.

void acb_dirichlet_zeta_rs_f_coeffs(acb_ptr f, const arb_t p, slong n, slong prec)

  Computes the coefficients \(F^{(j)}(p)\) for \(0 \le j < n\). Uses power series division. This method breaks down when \(p = \pm 1/2\) (which is not problem if s is an exact floating-point number).

void acb_dirichlet_zeta_rs_d_coeffs(arb_ptr d, const arb_t sigma, slong k, slong prec)

  Computes the coefficients \(d_j^{(k)}\) for \(0 \le j \le \lfloor 3k/2 \rfloor + 1\). On input, the array d must contain the coefficients for \(d_j^{(k-1)}\) unless \(k = 0\), and these coefficients will be updated in-place.

void acb_dirichlet_zeta_rs_bound(mag_t err, const acb_t s, slong K)

  Bounds the error term \(RS_K\) following Theorem 4.2 in Arias de Reyna.

void acb_dirichlet_zeta_rs_r(acb_t res, const acb_t s, slong K, slong prec)

  Computes \(\mathcal{R}(s)\) in the upper half plane. Uses precisely K asymptotic terms in the RS formula if this input parameter is positive; otherwise chooses the number of terms automatically based on s and the precision.

void acb_dirichlet_zeta_rs(acb_t res, const acb_t s, slong K, slong prec)

  Computes \(\zeta(s)\) using the Riemann-Siegel formula. Uses precisely K asymptotic terms in the RS formula if this input parameter is positive; otherwise chooses the number of terms automatically based on s and the precision.

void acb_dirichlet_zeta_jet_rs(acb_ptr res, const acb_t s, slong len, slong prec)

  Computes the first len terms of the Taylor series of the Riemann zeta function at s using the Riemann Siegel formula. This function currently only supports len = 1 or len = 2. A finite difference is used to compute the first derivative.

## Hurwitz zeta function

void acb_dirichlet_hurwitz(acb_t res, const acb_t s, const acb_t a, slong prec)

  Computes the Hurwitz zeta function \(\zeta(s, a)\). This function automatically delegates to the code for the Riemann zeta function when \(a = 1\). Some other special cases may also be handled by direct formulas. In general, Euler-Maclaurin summation is used.

## Hurwitz zeta function precomputation

type acb_dirichlet_hurwitz_precomp_struct

type acb_dirichlet_hurwitz_precomp_t

void acb_dirichlet_hurwitz_precomp_init(acb_dirichlet_hurwitz_precomp_t pre, const acb_t s, int deflate, slong A, slong K, slong N, slong prec)

  Precomputes a grid of Taylor polynomials for fast evaluation of \(\zeta(s,a)\) on \(a \in (0,1]\) with fixed s. A is the initial shift to apply to a, K is the number of Taylor terms, N is the number of grid points. The precomputation requires NK evaluations of the Hurwitz zeta function, and each subsequent evaluation requires 2K simple arithmetic operations (polynomial evaluation) plus A powers. As K grows, the error is at most \(O(1/(2AN)^K)\).

  This function can be called with A set to zero, in which case no Taylor series precomputation is performed. This means that evaluation will be identical to calling `acb_dirichlet_hurwitz()` directly.

  Otherwise, we require that A, K and N are all positive. For a finite error bound, we require \(K+\operatorname{re}(s) > 1\). To avoid an initial “bump” that steals precision and slows convergence, AN should be at least roughly as large as \(|s|\), e.g. it is a good idea to have at least \(AN > 0.5 |s|\).

  If deflate is set, the deflated Hurwitz zeta function is used, removing the pole at \(s = 1\).

void acb_dirichlet_hurwitz_precomp_init_num(acb_dirichlet_hurwitz_precomp_t pre, const acb_t s, int deflate, double num_eval, slong prec)

  Initializes pre, choosing the parameters A, K, and N automatically to minimize the cost of num_eval evaluations of the Hurwitz zeta function at argument s to precision prec.

void acb_dirichlet_hurwitz_precomp_clear(acb_dirichlet_hurwitz_precomp_t pre)

  Clears the precomputed data.

void acb_dirichlet_hurwitz_precomp_choose_param(ulong *A, ulong *K, ulong *N, const acb_t s, double num_eval, slong prec)

  Chooses precomputation parameters A, K and N to minimize the cost of num_eval evaluations of the Hurwitz zeta function at argument s to precision prec. If it is estimated that evaluating each Hurwitz zeta function from scratch would be better than performing a precomputation, A, K and N are all set to 0.

void acb_dirichlet_hurwitz_precomp_bound(mag_t res, const acb_t s, slong A, slong K, slong N)

  Computes an upper bound for the truncation error (not accounting for roundoff error) when evaluating \(\zeta(s,a)\) with precomputation parameters A, K, N, assuming that \(0 < a \le 1\). For details, see Algorithms for the Hurwitz zeta function.

void acb_dirichlet_hurwitz_precomp_eval(acb_t res, const acb_dirichlet_hurwitz_precomp_t pre, ulong p, ulong q, slong prec)

  Evaluates \(\zeta(s,p/q)\) using precomputed data, assuming that \(0 < p/q \le 1\).

## Lerch transcendent

void acb_dirichlet_lerch_phi_integral(acb_t res, const acb_t z, const acb_t s, const acb_t a, slong prec)
void acb_dirichlet_lerch_phi_direct(acb_t res, const acb_t z, const acb_t s, const acb_t a, slong prec)
void acb_dirichlet_lerch_phi(acb_t res, const acb_t z, const acb_t s, const acb_t a, slong prec)

  Computes the Lerch transcendent

  \[\Phi(z,s,a) = \sum_{k=0}^{\infty} \frac{z^k}{(k+a)^s}\]

  which is analytically continued for \(|z| \ge 1\).

  The direct version evaluates a truncation of the defining series. The integral version uses the Hankel contour integral

  \[\Phi(z,s,a) = -\frac{\Gamma(1-s)}{2 \pi i} \int_C \frac{(-t)^{s-1} e^{-a t}}{1 - z e^{-t}} dt\]

  where the path is deformed as needed to avoid poles and branch cuts of the integrand. The default method chooses an algorithm automatically and also checks for some special cases where the function can be expressed in terms of simpler functions (Hurwitz zeta, polylogarithms).

## Stieltjes constants

void acb_dirichlet_stieltjes(acb_t res, const fmpz_t n, const acb_t a, slong prec)

  Given a nonnegative integer n, sets res to the generalized Stieltjes constant \(\gamma_n(a)\) which is the coefficient in the Laurent series of the Hurwitz zeta function at the pole

  \[\zeta(s,a) = \frac{1}{s-1} + \sum_{n=0}^\infty \frac{(-1)^n}{n!} \gamma_n(a) (s-1)^n.\]

  With \(a = 1\), this gives the ordinary Stieltjes constants for the Riemann zeta function.

  This function uses an integral representation to permit fast computation for extremely large n [JB2018]. If n is moderate and the precision is high enough, it falls back to evaluating the Hurwitz zeta function of a power series and reading off the last coefficient.

  Note that for computing a range of values \(\gamma_0(a), \ldots, \gamma_n(a)\), it is generally more efficient to evaluate the Hurwitz zeta function series expansion once at \(s = 1\) than to call this function repeatedly, unless n is extremely large (at least several hundred).

## Dirichlet character evaluation

void acb_dirichlet_chi(acb_t res, const dirichlet_group_t G, const dirichlet_char_t chi, ulong n, slong prec)

  Sets res to \(\chi(n)\), the value of the Dirichlet character chi at the integer n.

void acb_dirichlet_chi_vec(acb_ptr v, const dirichlet_group_t G, const dirichlet_char_t chi, slong nv, slong prec)

  Compute the nv first Dirichlet values.

void acb_dirichlet_pairing(acb_t res, const dirichlet_group_t G, ulong m, ulong n, slong prec)

void acb_dirichlet_pairing_char(acb_t res, const dirichlet_group_t G, const dirichlet_char_t a, const dirichlet_char_t b, slong prec)

  Sets res to the value of the Dirichlet pairing \(\chi(m,n)\) at numbers \(m\) and \(n\). The second form takes two characters as input.

## Dirichlet character Gauss, Jacobi and theta sums

void acb_dirichlet_gauss_sum_naive(acb_t res, const dirichlet_group_t G, const dirichlet_char_t chi, slong prec)

void acb_dirichlet_gauss_sum_factor(acb_t res, const dirichlet_group_t G, const dirichlet_char_t chi, slong prec)

void acb_dirichlet_gauss_sum_order2(acb_t res, const dirichlet_group_t G, const dirichlet_char_t chi, slong prec)

void acb_dirichlet_gauss_sum_theta(acb_t res, const dirichlet_group_t G, const dirichlet_char_t chi, slong prec)

void acb_dirichlet_gauss_sum(acb_t res, const dirichlet_group_t G, const dirichlet_char_t chi, slong prec)

  Sets res to the Gauss sum

  \[G_q(a) = \sum_{x \bmod q} \chi_q(a, x) e^{\frac{2i\pi x}q}\]

  - the naive version computes the sum as defined.

  - the factor version writes it as a product of local Gauss sums by chinese remainder theorem.

  - the order2 version assumes chi is real and primitive and returns \(i^p\sqrt q\) where \(p\) is the parity of \(\chi\).

  - the theta version assumes that chi is primitive to obtain the Gauss sum by functional equation of the theta series at \(t=1\). An abort will be raised if the theta series vanishes at \(t=1\). Only 4 exceptional characters of conductor 300 and 600 are known to have this particularity, and none with primepower modulus.

  - the default version automatically combines the above methods.

  - the ui version only takes the Conrey number a as parameter.

void acb_dirichlet_jacobi_sum_naive(acb_t res, const dirichlet_group_t G, const dirichlet_char_t chi1, const dirichlet_char_t chi2, slong prec)

void acb_dirichlet_jacobi_sum_factor(acb_t res, const dirichlet_group_t G, const dirichlet_char_t chi1, const dirichlet_char_t chi2, slong prec)

void acb_dirichlet_jacobi_sum_gauss(acb_t res, const dirichlet_group_t G, const dirichlet_char_t chi1, const dirichlet_char_t chi2, slong prec)

void acb_dirichlet_jacobi_sum(acb_t res, const dirichlet_group_t G, const dirichlet_char_t chi1, const dirichlet_char_t chi2, slong prec)

void acb_dirichlet_jacobi_sum_ui(acb_t res, const dirichlet_group_t G, ulong a, ulong b, slong prec)

  Computes the Jacobi sum

  \[J_q(a,b) = \sum_{x \bmod q} \chi_q(a, x)\chi_q(b, 1-x)\]

  - the naive version computes the sum as defined.

  - the factor version writes it as a product of local Jacobi sums

  - the gauss version assumes \(ab\) is primitive and uses the formula \(J_q(a,b)G_q(ab) = G_q(a)G_q(b)\)

  - the default version automatically combines the above methods.

  - the ui version only takes the Conrey numbers a and b as parameters.

void acb_dirichlet_chi_theta_arb(acb_t res, const dirichlet_group_t G, const dirichlet_char_t chi, const arb_t t, slong prec)

void acb_dirichlet_ui_theta_arb(acb_t res, const dirichlet_group_t G, ulong a, const arb_t t, slong prec)

  Compute the theta series \(\Theta_q(a,t)\) for real argument \(t>0\). Beware that if \(t<1\) the functional equation

  \[t \theta(a,t) = \epsilon(\chi) \theta\left(\frac1a, \frac1t\right)\]

  should be used, which is not done automatically (to avoid recomputing the Gauss sum).

  We call theta series of a Dirichlet character the quadratic series

  \[\Theta_q(a) = \sum_{n\geq 0} \chi_q(a, n) n^p x^{n^2}\]

  where \(p\) is the parity of the character \(\chi_q(a,\cdot)\).

  For \(\Re(t)>0\) we write \(x(t)=\exp(-\frac{\pi}{N}t^2)\) and define

  \[\Theta_q(a,t) = \sum_{n\geq 0} \chi_q(a, n) x(t)^{n^2}.\]

ulong acb_dirichlet_theta_length(ulong q, const arb_t t, slong prec)

  Compute the number of terms to be summed in the theta series of argument t so that the tail is less than \(2^{-\mathrm{prec}}\).

void acb_dirichlet_qseries_arb_powers_naive(acb_t res, const arb_t x, int p, const ulong *a, const acb_dirichlet_roots_t z, slong len, slong prec)

void acb_dirichlet_qseries_arb_powers_smallorder(acb_t res, const arb_t x, int p, const ulong *a, const acb_dirichlet_roots_t z, slong len, slong prec)

  Compute the series \(\sum n^p z^{a_n} x^{n^2}\) for exponent list a, precomputed powers z and parity p (being 0 or 1).

  The naive version sums the series as defined, while the smallorder variant evaluates the series on the quotient ring by a cyclotomic polynomial before evaluating at the root of unity, ignoring its argument z.

## Discrete Fourier transforms

If \(f\) is a function \(\mathbb Z/q\mathbb Z\to \mathbb C\), its discrete Fourier transform is the function defined on Dirichlet characters mod \(q\) by

\[\hat f(\chi) = \sum_{x\mod q}\overline{\chi(x)}f(x)\]

See the acb_dft.h – Discrete Fourier transform module.

Here we take advantage of the Conrey isomorphism \(G \to \hat G\) to consider the Fourier transform on Conrey labels as

\[g(a) = \sum_{b\bmod q}\overline{\chi_q(a,b)}f(b)\]

void acb_dirichlet_dft_conrey(acb_ptr w, acb_srcptr v, const dirichlet_group_t G, slong prec)

  Compute the DFT of v using Conrey indices. This function assumes v and w are vectors of size G->phi_q, whose values correspond to a lexicographic ordering of Conrey logs (as obtained using `dirichlet_char_next()` or by `dirichlet_char_index()`).

  For example, if \(q=15\), the Conrey elements are stored in following order

  index | log = [e,f] | number = 7^e 11^f
  --- | --- | ---
  0 | [0, 0] | 1
  1 | [0, 1] | 7
  2 | [0, 2] | 4
  3 | [0, 3] | 13
  4 | [0, 4] | 1
  5 | [1, 0] | 11
  6 | [1, 1] | 2
  7 | [1, 2] | 14
  8 | [1, 3] | 8
  9 | [1, 4] | 11

void acb_dirichlet_dft(acb_ptr w, acb_srcptr v, const dirichlet_group_t G, slong prec)

  Compute the DFT of v using Conrey numbers. This function assumes v and w are vectors of size G->q. All values at index not coprime to G->q are ignored.

## Dirichlet L-functions

void acb_dirichlet_root_number_theta(acb_t res, const dirichlet_group_t G, const dirichlet_char_t chi, slong prec)

void acb_dirichlet_root_number(acb_t res, const dirichlet_group_t G, const dirichlet_char_t chi, slong prec)

  Sets res to the root number \(\epsilon(\chi)\) for a primitive character chi, which appears in the functional equation (where \(p\) is the parity of \(\chi\)):

  \[\left(\frac{q}{\pi}\right)^{\frac{s+p}2}\Gamma\left(\frac{s+p}2\right) L(s, \chi) = \epsilon(\chi) \left(\frac{q}{\pi}\right)^{\frac{1-s+p}2}\Gamma\left(\frac{1-s+p}2\right) L(1 - s, \overline\chi)\]

  - The theta variant uses the evaluation at \(t=1\) of the Theta series.

  - The default version computes it via the gauss sum.

void acb_dirichlet_l_hurwitz(acb_t res, const acb_t s, const acb_dirichlet_hurwitz_precomp_t precomp, const dirichlet_group_t G, const dirichlet_char_t chi, slong prec)

  Computes \(L(s,\chi)\) using decomposition in terms of the Hurwitz zeta function

  \[L(s,\chi) = q^{-s}\sum_{k=1}^q \chi(k) \,\zeta\!\left(s,\frac kq\right).\]

  If \(s = 1\) and \(\chi\) is non-principal, the deflated Hurwitz zeta function is used to avoid poles.

  If precomp is NULL, each Hurwitz zeta function value is computed directly. If a pre-initialized precomp object is provided, this will be used instead to evaluate the Hurwitz zeta function.

void acb_dirichlet_l_euler_product(acb_t res, const acb_t s, const dirichlet_group_t G, const dirichlet_char_t chi, slong prec)

void _acb_dirichlet_euler_product_real_ui(arb_t res, ulong s, const signed char *chi, int mod, int reciprocal, slong prec)

  Computes \(L(s,\chi)\) directly using the Euler product. This is efficient if s has large positive real part. As implemented, this function only gives a finite result if \(\operatorname{re}(s) \ge 2\).

  An error bound is computed via `mag_hurwitz_zeta_uiui()`. If s is complex, replace it with its real part. Since

  \[\frac{1}{L(s,\chi)} = \prod_{p} \left(1 - \frac{\chi(p)}{p^s}\right) = \sum_{k=1}^{\infty} \frac{\mu(k)\chi(k)}{k^s}\]

  and the truncated product gives all smooth-index terms in the series, we have

  \[\left|\prod_{p < N} \left(1 - \frac{\chi(p)}{p^s}\right) - \frac{1}{L(s,\chi)}\right| \le \sum_{k=N}^{\infty} \frac{1}{k^s} = \zeta(s,N).\]

  The underscore version specialized for integer s assumes that \(\chi\) is a real Dirichlet character given by the explicit list chi of character values at 0, 1, …, mod - 1. If reciprocal is set, it computes \(1 / L(s,\chi)\) (this is faster if the reciprocal can be used directly).

void acb_dirichlet_l(acb_t res, const acb_t s, const dirichlet_group_t G, const dirichlet_char_t chi, slong prec)

  Computes \(L(s,\chi)\) using a default choice of algorithm.

void acb_dirichlet_l_fmpq(acb_t res, const fmpq_t s, const dirichlet_group_t G, const dirichlet_char_t chi, slong prec)
void acb_dirichlet_l_fmpq_afe(acb_t res, const fmpq_t s, const dirichlet_group_t G, const dirichlet_char_t chi, slong prec)

  Computes \(L(s,\chi)\) where s is a rational number. The afe version uses the approximate functional equation; the default version chooses an algorithm automatically.

void acb_dirichlet_l_vec_hurwitz(acb_ptr res, const acb_t s, const acb_dirichlet_hurwitz_precomp_t precomp, const dirichlet_group_t G, slong prec)

  Compute all values \(L(s,\chi)\) for \(\chi\) mod \(q\), using the Hurwitz zeta function and a discrete Fourier transform. The output res is assumed to have length G->phi_q and values are stored by lexicographically ordered Conrey logs. See `acb_dirichlet_dft_conrey()`.

  If precomp is NULL, each Hurwitz zeta function value is computed directly. If a pre-initialized precomp object is provided, this will be used instead to evaluate the Hurwitz zeta function.

void acb_dirichlet_l_jet(acb_ptr res, const acb_t s, const dirichlet_group_t G, const dirichlet_char_t chi, int deflate, slong len, slong prec)

  Computes the Taylor expansion of \(L(s,\chi)\) to length len, i.e. \(L(s), L'(s), \ldots, L^{(len-1)}(s) / (len-1)!\). If deflate is set, computes the expansion of

  \[L(s,\chi) - \frac{\sum_{k=1}^q \chi(k)}{(s-1)q}\]

  instead. If chi is a principal character, then this has the effect of subtracting the pole with residue \(\sum_{k=1}^q \chi(k) = \phi(q) / q\) that is located at \(s = 1\). In particular, when evaluated at \(s = 1\), this gives the regular part of the Laurent expansion. When chi is non-principal, deflate has no effect.

void _acb_dirichlet_l_series(acb_ptr res, acb_srcptr s, slong slen, const dirichlet_group_t G, const dirichlet_char_t chi, int deflate, slong len, slong prec)

void acb_dirichlet_l_series(acb_poly_t res, const acb_poly_t s, const dirichlet_group_t G, const dirichlet_char_t chi, int deflate, slong len, slong prec)

  Sets res to the power series \(L(s,\chi)\) where s is a given power series, truncating the result to length len. See `acb_dirichlet_l_jet()` for the meaning of the deflate flag.

## Hardy Z-functions

For convenience, setting both G and chi to NULL in the following methods selects the Riemann zeta function.

Currently, these methods require chi to be a primitive character.

void acb_dirichlet_hardy_theta(acb_ptr res, const acb_t t, const dirichlet_group_t G, const dirichlet_char_t chi, slong len, slong prec)

  Computes the phase function used to construct the Z-function. We have

  \[\theta(t) = -\frac{t}{2} \log(\pi/q) - \frac{i \log(\epsilon)}{2} + \frac{\log \Gamma((s+\delta)/2) - \log \Gamma((1-s+\delta)/2)}{2i}\]

  where \(s = 1/2+it\), \(\delta\) is the parity of chi, and \(\epsilon\) is the root number as computed by `acb_dirichlet_root_number()`. The first len terms in the Taylor expansion are written to the output.

void acb_dirichlet_hardy_z(acb_ptr res, const acb_t t, const dirichlet_group_t G, const dirichlet_char_t chi, slong len, slong prec)

  Computes the Hardy Z-function, also known as the Riemann-Siegel Z-function \(Z(t) = e^{i \theta(t)} L(1/2+it)\), which is real-valued for real t. The first len terms in the Taylor expansion are written to the output.

void _acb_dirichlet_hardy_theta_series(acb_ptr res, acb_srcptr t, slong tlen, const dirichlet_group_t G, const dirichlet_char_t chi, slong len, slong prec)

void acb_dirichlet_hardy_theta_series(acb_poly_t res, const acb_poly_t t, const dirichlet_group_t G, const dirichlet_char_t chi, slong len, slong prec)

  Sets res to the power series \(\theta(t)\) where t is a given power series, truncating the result to length len.

void _acb_dirichlet_hardy_z_series(acb_ptr res, acb_srcptr t, slong tlen, const dirichlet_group_t G, const dirichlet_char_t chi, slong len, slong prec)

void acb_dirichlet_hardy_z_series(acb_poly_t res, const acb_poly_t t, const dirichlet_group_t G, const dirichlet_char_t chi, slong len, slong prec)

  Sets res to the power series \(Z(t)\) where t is a given power series, truncating the result to length len.

## Gram points

void acb_dirichlet_gram_point(arb_t res, const fmpz_t n, const dirichlet_group_t G, const dirichlet_char_t chi, slong prec)

  Sets res to the n-th Gram point \(g_n\), defined as the unique solution in \([7, \infty)\) of \(\theta(g_n) = \pi n\). Currently only the Gram points corresponding to the Riemann zeta function are supported and G and chi must both be set to NULL. Requires \(n \ge -1\).

## Riemann zeta function zeros

The following functions for counting and isolating zeros of the Riemann zeta function use the ideas from the implementation of Turing’s method in mpmath [Joh2018b] by Juan Arias de Reyna, described in [Ari2012].

ulong acb_dirichlet_turing_method_bound(const fmpz_t p)

  Computes an upper bound B for the minimum number of consecutive good Gram blocks sufficient to count nontrivial zeros of the Riemann zeta function using Turing’s method [Tur1953] as updated by [Leh1970], [Bre1979], and [Tru2011].

  Let \(N(T)\) denote the number of zeros (counted according to their multiplicities) of \(\zeta(s)\) in the region \(0 < \operatorname{Im}(s) \le T\). If at least B consecutive Gram blocks with union \([g_n, g_p)\) satisfy Rosser’s rule, then \(N(g_n) \le n + 1\) and \(N(g_p) \ge p + 1\).

int _acb_dirichlet_definite_hardy_z(arb_t res, const arf_t t, slong *pprec)

  Sets res to the Hardy Z-function \(Z(t)\). The initial precision (* pprec) is increased as necessary to determine the sign of \(Z(t)\). The sign is returned.

void _acb_dirichlet_isolate_gram_hardy_z_zero(arf_t a, arf_t b, const fmpz_t n)

  Uses Gram’s law to compute an interval \((a, b)\) that contains the n-th zero of the Hardy Z-function and no other zero. Requires \(1 \le n \le 126\).

void _acb_dirichlet_isolate_rosser_hardy_z_zero(arf_t a, arf_t b, const fmpz_t n)

  Uses Rosser’s rule to compute an interval \((a, b)\) that contains the n-th zero of the Hardy Z-function and no other zero. Requires \(1 \le n \le 13999526\).

void _acb_dirichlet_isolate_turing_hardy_z_zero(arf_t a, arf_t b, const fmpz_t n)

  Computes an interval \((a, b)\) that contains the n-th zero of the Hardy Z-function and no other zero, following Turing’s method. Requires \(n \ge 2\).

void acb_dirichlet_isolate_hardy_z_zero(arf_t a, arf_t b, const fmpz_t n)

  Computes an interval \((a, b)\) that contains the n-th zero of the Hardy Z-function and contains no other zero, using the most appropriate underscore version of this function. Requires \(n \ge 1\).

void _acb_dirichlet_refine_hardy_z_zero(arb_t res, const arf_t a, const arf_t b, slong prec)

  Sets res to the unique zero of the Hardy Z-function in the interval \((a, b)\).

void acb_dirichlet_hardy_z_zero(arb_t res, const fmpz_t n, slong prec)

  Sets res to the n-th zero of the Hardy Z-function, requiring \(n \ge 1\).

void acb_dirichlet_hardy_z_zeros(arb_ptr res, const fmpz_t n, slong len, slong prec)

  Sets the entries of res to len consecutive zeros of the Hardy Z-function, beginning with the n-th zero. Requires positive n.

void acb_dirichlet_zeta_zero(acb_t res, const fmpz_t n, slong prec)

  Sets res to the n-th nontrivial zero of \(\zeta(s)\), requiring \(n \ge 1\).

void acb_dirichlet_zeta_zeros(acb_ptr res, const fmpz_t n, slong len, slong prec)

  Sets the entries of res to len consecutive nontrivial zeros of \(\zeta(s)\) beginning with the n-th zero. Requires positive n.

void _acb_dirichlet_exact_zeta_nzeros(fmpz_t res, const arf_t t)

void acb_dirichlet_zeta_nzeros(arb_t res, const arb_t t, slong prec)

  Compute the number of zeros (counted according to their multiplicities) of \(\zeta(s)\) in the region \(0 < \operatorname{Im}(s) \le t\).

void acb_dirichlet_backlund_s(arb_t res, const arb_t t, slong prec)

  Compute \(S(t) = \frac{1}{\pi}\operatorname{arg}\zeta(\frac{1}{2} + it)\) where the argument is defined by continuous variation of \(s\) in \(\zeta(s)\) starting at \(s = 2\), then vertically to \(s = 2 + it\), then horizontally to \(s = \frac{1}{2} + it\). In particular \(\operatorname{arg}\) in this context is not the principal value of the argument, and it cannot be computed directly by `acb_arg()`. In practice \(S(t)\) is computed as \(S(t) = N(t) - \frac{1}{\pi}\theta(t) - 1\) where \(N(t)\) is `acb_dirichlet_zeta_nzeros()` and \(\theta(t)\) is `acb_dirichlet_hardy_theta()`.

void acb_dirichlet_backlund_s_bound(mag_t res, const arb_t t)

  Compute an upper bound for \(|S(t)|\) quickly. Theorem 1 and the bounds in (1.2) in [Tru2014] are used.

void acb_dirichlet_zeta_nzeros_gram(fmpz_t res, const fmpz_t n)

  Compute \(N(g_n)\). That is, compute the number of zeros (counted according to their multiplicities) of \(\zeta(s)\) in the region \(0 < \operatorname{Im}(s) \le g_n\) where \(g_n\) is the n-th Gram point. Requires \(n \ge -1\).

slong acb_dirichlet_backlund_s_gram(const fmpz_t n)

  Compute \(S(g_n)\) where \(g_n\) is the n-th Gram point. Requires \(n \ge -1\).

## Riemann zeta function zeros (Platt’s method)

The following functions related to the Riemann zeta function use the ideas and formulas described by David J. Platt in [Pla2017].

void acb_dirichlet_platt_scaled_lambda(arb_t res, const arb_t t, slong prec)

  Compute \(\Lambda(t) e^{\pi t/4}\) where

  \[\Lambda(t) = \pi^{-\frac{it}{2}} \Gamma\left(\frac{\frac{1}{2}+it}{2}\right) \zeta\left(\frac{1}{2} + it\right)\]

  is defined in the beginning of section 3 of [Pla2017]. As explained in [Pla2011] this function has the same zeros as \(\zeta(1/2 + it)\) and is real-valued by the functional equation, and the exponential factor is designed to counteract the decay of the gamma factor as \(t\) increases.

void acb_dirichlet_platt_scaled_lambda_vec(arb_ptr res, const fmpz_t T, slong A, slong B, slong prec)

void acb_dirichlet_platt_multieval(arb_ptr res, const fmpz_t T, slong A, slong B, const arb_t h, const fmpz_t J, slong K, slong sigma, slong prec)

void acb_dirichlet_platt_multieval_threaded(arb_ptr res, const fmpz_t T, slong A, slong B, const arb_t h, const fmpz_t J, slong K, slong sigma, slong prec)

  Compute `acb_dirichlet_platt_scaled_lambda()` at \(N=AB\) points on a grid, following the notation of [Pla2017]. The first point on the grid is \(T - B/2\) and the distance between grid points is \(1/A\). The product \(N=AB\) must be an even integer. The multieval versions evaluate the function at all points on the grid simultaneously using discrete Fourier transforms, and they require the four additional tuning parameters h, J, K, and sigma. The threaded multieval version splits the computation over the number of threads returned by flint_get_num_threads(), while the default multieval version chooses whether to use multithreading automatically.

void acb_dirichlet_platt_ws_interpolation(arb_t res, arf_t deriv, const arb_t t0, arb_srcptr p, const fmpz_t T, slong A, slong B, slong Ns_max, const arb_t H, slong sigma, slong prec)

  Compute `acb_dirichlet_platt_scaled_lambda()` at t0 by Gaussian-windowed Whittaker-Shannon interpolation of points evaluated by `acb_dirichlet_platt_scaled_lambda_vec()`. The derivative is also approximated if the output parameter deriv is not NULL. Ns_max defines the maximum number of supporting points to be used in the interpolation on either side of t0. H is the standard deviation of the Gaussian window centered on t0 to be applied before the interpolation. sigma is an odd positive integer tuning parameter \(\sigma \in 2\mathbb{Z}_{>0}+1\) used in computing error bounds.

slong _acb_dirichlet_platt_local_hardy_z_zeros(arb_ptr res, const fmpz_t n, slong len, const fmpz_t T, slong A, slong B, const arb_t h, const fmpz_t J, slong K, slong sigma_grid, slong Ns_max, const arb_t H, slong sigma_interp, slong prec)

slong acb_dirichlet_platt_local_hardy_z_zeros(arb_ptr res, const fmpz_t n, slong len, slong prec)

slong acb_dirichlet_platt_hardy_z_zeros(arb_ptr res, const fmpz_t n, slong len, slong prec)

  Sets at most the first len entries of res to consecutive zeros of the Hardy Z-function starting with the n-th zero. The number of obtained consecutive zeros is returned. The first two function variants each make a single call to Platt’s grid evaluation of the scaled Lambda function, whereas the third variant performs as many evaluations as necessary to obtain len consecutive zeros. The final several parameters of the underscored local variant have the same meanings as in the functions `acb_dirichlet_platt_multieval()` and `acb_dirichlet_platt_ws_interpolation()`. The non-underscored variants currently expect \(10^4 \leq n \leq 10^{23}\). The user has the option of multi-threading through flint_set_num_threads(numthreads).

slong acb_dirichlet_platt_zeta_zeros(acb_ptr res, const fmpz_t n, slong len, slong prec)

  Sets at most the first len entries of res to consecutive zeros of the Riemann zeta function starting with the n-th zero. The number of obtained consecutive zeros is returned. It currently expects \(10^4 \leq n \leq 10^{23}\). The user has the option of multi-threading through flint_set_num_threads(numthreads).