Class for computing sums over zeros of motivic \(L\)-functions#

All computations are done to double precision.

AUTHORS:

Simon Spicer (2014-10): first version

sage.lfunctions.zero_sums.LFunctionZeroSum(X, *args, **kwds)#

Constructor for the LFunctionZeroSum class.

INPUT:

X – A motivic object. Currently only implemented for X = an elliptic curve over the rational numbers.

OUTPUT:

An LFunctionZeroSum object.

EXAMPLES:

sage: E = EllipticCurve("389a")
sage: Z = LFunctionZeroSum(E); Z
Zero sum estimator for L-function attached to Elliptic Curve defined by y^2 + y = x^3 + x^2 - 2*x over Rational Field

class sage.lfunctions.zero_sums.LFunctionZeroSum_EllipticCurve#

Bases: LFunctionZeroSum_abstract

Subclass for computing certain sums over zeros of an elliptic curve L-function without having to determine the zeros themselves.

analytic_rank_upper_bound(max_Delta=None, adaptive=True, root_number='compute', bad_primes=None, ncpus=None)#

Return an upper bound for the analytic rank of the L-function \(L_E(s)\) attached to self, conditional on the Generalized Riemann Hypothesis, via computing the zero sum \(\sum_{\gamma} f(\Delta\gamma)\), where \(\gamma\) ranges over the imaginary parts of the zeros of \(L(E,s)\) along the critical strip, \(f(x) = \left(\frac{\sin(\pi x)}{\pi x}\right)^2\), and \(\Delta\) is the tightness parameter whose maximum value is specified by max_Delta.

This computation can be run on curves with very large conductor (so long as the conductor is known or quickly computable) when Delta is not too large (see below).

Uses Bober’s rank bounding method as described in [Bob2013].

INPUT:

max_Delta – (default: None) If not None, a positive real value specifying the maximum Delta value used in the zero sum; larger values of Delta yield better bounds - but runtime is exponential in Delta. If left as None, Delta is set to \(\min\left\{\frac{1}{\pi}\left(\log(N+1000)/2-\log(2\pi)-\eta\right), 2.5\right\}\), where \(N\) is the conductor of the curve attached to self, and \(\eta\) is the Euler-Mascheroni constant \(= 0.5772...\); the crossover point is at conductor ~8.3*10^8. For the former value, empirical results show that for about 99.7% of all curves the returned value is the actual analytic rank.
adaptive – (default: True) Boolean
- If True, the computation is first run with small and then successively larger Delta values up to max_Delta. If at any point the computed bound is 0 (or 1 when root_number is -1 or True), the computation halts and that value is returned; otherwise the minimum of the computed bounds is returned.
- If False, the computation is run a single time with Delta=max_Delta, and the resulting bound returned.
root_number – (default: “compute”) String or integer
- "compute" – the root number of self is computed and used to (possibly) lower the analytic rank estimate by 1.
- "ignore" – the above step is omitted
- 1 – this value is assumed to be the root number of self. This is passable so that rank estimation can be done for curves whose root number has been precomputed.
- -1 – this value is assumed to be the root number of self. This is passable so that rank estimation can be done for curves whose root number has been precomputed.
bad_primes – (default: None) If not None, a list of the primes of bad reduction for the curve attached to self. This is passable so that rank estimation can be done for curves of large conductor whose bad primes have been precomputed.
ncpus – (default: None) If not None, a positive integer defining the maximum number of CPUs to be used for the computation. If left as None, the maximum available number of CPUs will be used. Note: Multiple processors will only be used for Delta values >= 1.75.

Note

Output will be incorrect if the incorrect root number is specified.

Warning

Zero sum computation time is exponential in the tightness parameter \(\Delta\), roughly doubling for every increase of 0.1 thereof. Using \(\Delta=1\) (and adaptive=False) will yield a runtime of a few milliseconds; \(\Delta=2\) takes a few seconds, and \(\Delta=3\) may take upwards of an hour. Increase beyond this at your own risk!

OUTPUT:

A non-negative integer greater than or equal to the analytic rank of self. If the returned value is 0 or 1 (the latter if parity is not False), then this is the true analytic rank of self.

Note

If you use set_verbose(1), extra information about the computation will be printed.

See also

LFunctionZeroSum() EllipticCurve.root_number() set_verbose()

EXAMPLES:

For most elliptic curves with small conductor the central zero(s) of \(L_E(s)\) are fairly isolated, so small values of \(\Delta\) will yield tight rank estimates.

sage: E = EllipticCurve("11a")
sage: E.rank()
0
sage: Z = LFunctionZeroSum(E)
sage: Z.analytic_rank_upper_bound(max_Delta=1,ncpus=1)
0

sage: E = EllipticCurve([-39,123])
sage: E.rank()
1
sage: Z = LFunctionZeroSum(E)
sage: Z.analytic_rank_upper_bound(max_Delta=1)
1

This is especially true for elliptic curves with large rank.

sage: for r in range(9):
....:     E = elliptic_curves.rank(r)[0]
....:     print((r, E.analytic_rank_upper_bound(max_Delta=1,
....:     adaptive=False,root_number="ignore")))
(0, 0)
(1, 1)
(2, 2)
(3, 3)
(4, 4)
(5, 5)
(6, 6)
(7, 7)
(8, 8)

However, some curves have \(L\)-functions with low-lying zeroes, and for these larger values of \(\Delta\) must be used to get tight estimates.

sage: E = EllipticCurve("974b1")
sage: r = E.rank(); r
0
sage: Z = LFunctionZeroSum(E)
sage: Z.analytic_rank_upper_bound(max_Delta=1,root_number="ignore")
1
sage: Z.analytic_rank_upper_bound(max_Delta=1.3,root_number="ignore")
0

Knowing the root number of E allows us to use smaller Delta values to get tight bounds, thus speeding up runtime considerably.

sage: Z.analytic_rank_upper_bound(max_Delta=0.6,root_number="compute")
0

The are a small number of curves which have pathologically low-lying zeroes. For these curves, this method will produce a bound that is strictly larger than the analytic rank, unless very large values of Delta are used. The following curve (“256944c1” in the Cremona tables) is a rank 0 curve with a zero at 0.0256…; the smallest Delta value for which the zero sum is strictly less than 2 is ~2.815.

sage: E = EllipticCurve([0, -1, 0, -7460362000712, -7842981500851012704])
sage: N,r = E.conductor(),E.analytic_rank(); N, r
(256944, 0)
sage: E.analytic_rank_upper_bound(max_Delta=1,adaptive=False)
2
sage: E.analytic_rank_upper_bound(max_Delta=2,adaptive=False)
2

This method is can be called on curves with large conductor.

sage: E = EllipticCurve([-2934,19238])
sage: Z = LFunctionZeroSum(E)
sage: Z.analytic_rank_upper_bound()
1

And it can bound rank on curves with very large conductor, so long as you know beforehand/can easily compute the conductor and primes of bad reduction less than \(e^{2\pi\Delta}\). The example below is of the rank 28 curve discovered by Elkies that is the elliptic curve of (currently) largest known rank.

sage: a4 = -20067762415575526585033208209338542750930230312178956502
sage: a6 = 34481611795030556467032985690390720374855944359319180361266008296291939448732243429
sage: E = EllipticCurve([1,-1,1,a4,a6])
sage: bad_primes = [2,3,5,7,11,13,17,19,48463]
sage: N = 3455601108357547341532253864901605231198511505793733138900595189472144724781456635380154149870961231592352897621963802238155192936274322687070
sage: Z = LFunctionZeroSum(E,N)
sage: Z.analytic_rank_upper_bound(max_Delta=2.37,adaptive=False, # long time
....: root_number=1,bad_primes=bad_primes,ncpus=2)               # long time
32

cn(n)#

Return the nth Dirichlet coefficient of the logarithmic derivative of the L-function attached to self, shifted so that the critical line lies on the imaginary axis.

The returned value is zero if \(n\) is not a perfect prime power; when \(n=p^e\) for \(p\) a prime of bad reduction it is \(-a_p^e log(p)/p^e\), where \(a_p\) is \(+1, -1\) or \(0\) according to the reduction type of \(p\); and when \(n=p^e\) for a prime \(p\) of good reduction, the value is \(-(\alpha_p^e + \beta_p^e) \log(p)/p^e\), where \(\alpha_p\) and \(\beta_p\) are the two complex roots of the characteristic equation of Frobenius at \(p\) on \(E\).

INPUT:

n – non-negative integer

OUTPUT:

A real number which (by Hasse’s Theorem) is at most \(2\frac{log(n)}{\sqrt{n}}\) in magnitude.

EXAMPLES:

sage: E = EllipticCurve("11a")
sage: Z = LFunctionZeroSum(E)
sage: for n in range(12): print((n, Z.cn(n))) # tol 1.0e-13
(0, 0.0)
(1, 0.0)
(2, 0.6931471805599453)
(3, 0.3662040962227033)
(4, 0.0)
(5, -0.32188758248682003)
(6, 0.0)
(7, 0.555974328301518)
(8, -0.34657359027997264)
(9, 0.6103401603711721)
(10, 0.0)
(11, -0.21799047934530644)

elliptic_curve()#

Return the elliptic curve associated with self.

EXAMPLES:

sage: E = EllipticCurve([23,100])
sage: Z = LFunctionZeroSum(E)
sage: Z.elliptic_curve()
Elliptic Curve defined by y^2 = x^3 + 23*x + 100 over Rational Field

lseries()#

Return the \(L\)-series associated with self.

EXAMPLES:

sage: E = EllipticCurve([23,100])
sage: Z = LFunctionZeroSum(E)
sage: Z.lseries()
Complex L-series of the Elliptic Curve defined by y^2 = x^3 + 23*x + 100 over Rational Field

class sage.lfunctions.zero_sums.LFunctionZeroSum_abstract#

Bases: SageObject

Abstract class for computing certain sums over zeros of a motivic L-function without having to determine the zeros themselves.

C0(include_euler_gamma=True)#

Return the constant term of the logarithmic derivative of the completed \(L\)-function attached to self.

This is equal to \(-\eta + \log(N)/2 - \log(2\pi)\), where \(\eta\) is the Euler-Mascheroni constant \(= 0.5772...\) and \(N\) is the level of the form attached to self.

INPUT:

include_euler_gamma – bool (default: True); if set to False, return the constant \(\log(N)/2 - \log(2\pi)\), i.e., do not subtract off the Euler-Mascheroni constant.

EXAMPLES:

sage: E = EllipticCurve("389a")
sage: Z = LFunctionZeroSum(E)
sage: Z.C0() # tol 1.0e-13
0.5666969404983447
sage: Z.C0(include_euler_gamma=False) # tol 1.0e-13
1.1439126053998776

cnlist(n, python_floats=False)#

Return a list of Dirichlet coefficient of the logarithmic derivative of the \(L\)-function attached to self, shifted so that the critical line lies on the imaginary axis, up to and including n.

The i-th element of the returned list is a[i].

INPUT:

n – non-negative integer
python_floats – bool (default: False); if True return a list of Python floats instead of Sage Real Double Field elements.

OUTPUT:

A list of real numbers

See also

cn()

Todo

Speed this up; make more efficient

EXAMPLES:

sage: E = EllipticCurve("11a")
sage: Z = LFunctionZeroSum(E)
sage: cnlist = Z.cnlist(11)
sage: for n in range(12): print((n, cnlist[n])) # tol 1.0e-13
(0, 0.0)
(1, 0.0)
(2, 0.6931471805599453)
(3, 0.3662040962227033)
(4, 0.0)
(5, -0.32188758248682003)
(6, 0.0)
(7, 0.555974328301518)
(8, -0.34657359027997264)
(9, 0.6103401603711721)
(10, 0.0)
(11, -0.21799047934530644)

completed_logarithmic_derivative(s, num_terms=10000)#

Compute the value of the completed logarithmic derivative \(\frac{\Lambda^{\prime}}{\Lambda}\) at the point s to low precision, where \(\Lambda = N^{s/2}(2\pi)^s \Gamma(s) L(s)\) and \(L\) is the \(L\)-function attached to self.

Warning

This is computed naively by evaluating the Dirichlet series for \(\frac{L^{\prime}}{L}\); the convergence thereof is controlled by the distance of s from the critical strip \(0.5<=\Re(s)<=1.5\). You may use this method to attempt to compute values inside the critical strip; however, results are then not guaranteed to be correct to any number of digits.

INPUT:

s – Real or complex value
num_terms – (default: 10000) the maximum number of terms summed in the Dirichlet series.

OUTPUT:

A tuple (z,err), where z is the computed value, and err is an upper bound on the truncation error in this value introduced by truncating the Dirichlet sum.

Note

For the default term cap of 10000, a value accurate to all 53 bits of a double precision floating point number is only guaranteed when \(|\Re(s-1)|>4.58\), although in practice inputs closer to the critical strip will still yield computed values close to the true value.

See also

logarithmic_derivative()

EXAMPLES:

sage: E = EllipticCurve([23,100])
sage: Z = LFunctionZeroSum(E)
sage: Z.completed_logarithmic_derivative(3) # tol 1.0e-13
(6.64372066048195, 6.584671359095225e-06)

Complex values are handled. The function is odd about s=1, so the value at 2-s should be minus the value at s.

sage: Z.completed_logarithmic_derivative(complex(-2.2,1)) # tol 1.0e-13
(-6.898080633125154 + 0.22557015394248361*I, 5.623853049808912e-11)
sage: Z.completed_logarithmic_derivative(complex(4.2,-1)) # tol 1.0e-13
(6.898080633125154 - 0.22557015394248361*I, 5.623853049808912e-11)

digamma(s, include_constant_term=True)#

Return the digamma function \(\digamma(s)\) on the complex input s.

This is given by \(\digamma(s) = -\eta + \sum_{k=1}^{\infty} \frac{s-1}{k(k+s-1)}\), where \(\eta\) is the Euler-Mascheroni constant \(=0.5772156649\ldots\).

This function is needed in the computing the logarithmic derivative of the \(L\)-function attached to self.

INPUT:

s – A complex number
include_constant_term – (default: True) boolean; if set to False, only the value of the sum over \(k\) is returned without subtracting off the Euler-Mascheroni constant, i.e. the returned value is equal to \(\sum_{k=1}^{\infty} \frac{s-1}{k(k+s-1)}\).

OUTPUT:

A real double precision number if the input is real and not a negative integer; Infinity if the input is a negative integer, and a complex number otherwise.

EXAMPLES:

sage: Z = LFunctionZeroSum(EllipticCurve("37a"))
sage: Z.digamma(3.2) # tol 1.0e-13
0.9988388912865993
sage: Z.digamma(3.2,include_constant_term=False) # tol 1.0e-13
1.576054556188132
sage: Z.digamma(1+I) # tol 1.0e-13
0.09465032062247625 + 1.076674047468581*I
sage: Z.digamma(-2)
+Infinity

Evaluating the sum without the constant term at the positive integers n returns the (n-1)th harmonic number.

sage: Z.digamma(3,include_constant_term=False)
1.5
sage: Z.digamma(6,include_constant_term=False)
2.283333333333333

level()#

Return the level of the form attached to self.

If self was constructed from an elliptic curve, then this is equal to the conductor of \(E\).

EXAMPLES:

sage: E = EllipticCurve("389a")
sage: Z = LFunctionZeroSum(E)
sage: Z.level()
389

logarithmic_derivative(s, num_terms=10000, as_interval=False)#

Compute the value of the logarithmic derivative \(\frac{L^{\prime}}{L}\) at the point s to low precision, where \(L\) is the \(L\)-function attached to self.

Warning

The value is computed naively by evaluating the Dirichlet series for \(\frac{L^{\prime}}{L}\); convergence is controlled by the distance of s from the critical strip \(0.5<=\Re(s)<=1.5\). You may use this method to attempt to compute values inside the critical strip; however, results are then not guaranteed to be correct to any number of digits.

INPUT:

s – Real or complex value
num_terms – (default: 10000) the maximum number of terms summed in the Dirichlet series.

OUTPUT:

A tuple (z,err), where z is the computed value, and err is an upper bound on the truncation error in this value introduced by truncating the Dirichlet sum.

Note

EXAMPLES:

sage: E = EllipticCurve([23,100])
sage: Z = LFunctionZeroSum(E)
sage: Z.logarithmic_derivative(10) # tol 1.0e-13
(5.648066742632698e-05, 1.0974102859764345e-34)
sage: Z.logarithmic_derivative(2.2) # tol 1.0e-13
(0.5751257063594758, 0.024087912696974387)

Increasing the number of terms should see the truncation error decrease.

sage: Z.logarithmic_derivative(2.2,num_terms=50000) # long time # rel tol 1.0e-14
(0.5751579645060139, 0.008988775519160675)

Attempting to compute values inside the critical strip gives infinite error.

sage: Z.logarithmic_derivative(1.3) # tol 1.0e-13
(5.442994413920786, +Infinity)

Complex inputs and inputs to the left of the critical strip are allowed.

sage: Z.logarithmic_derivative(complex(3,-1)) # tol 1.0e-13
(0.04764548578052381 + 0.16513832809989326*I, 6.584671359095225e-06)
sage: Z.logarithmic_derivative(complex(-3,-1.1)) # tol 1.0e-13
(-13.908452173241546 + 2.591443099074753*I, 2.7131584736258447e-14)

The logarithmic derivative has poles at the negative integers.

sage: Z.logarithmic_derivative(-3) # tol 1.0e-13
(-Infinity, 2.7131584736258447e-14)

ncpus(n=None)#

Set or return the number of CPUs to be used in parallel computations.

If called with no input, the number of CPUs currently set is returned; else this value is set to n. If n is 0 then the number of CPUs is set to the max available.

INPUT:

n – (default: None) If not None, a nonnegative integer

OUTPUT:

If n is not None, returns a positive integer

EXAMPLES:

sage: Z = LFunctionZeroSum(EllipticCurve("389a"))
sage: Z.ncpus()
1
sage: Z.ncpus(2)
sage: Z.ncpus()
2

The following output will depend on the system that Sage is running on.

sage: Z.ncpus(0)
sage: Z.ncpus()            # random
4

weight()#

Return the weight of the form attached to self.

If self was constructed from an elliptic curve, then this is 2.

EXAMPLES:

sage: E = EllipticCurve("389a")
sage: Z = LFunctionZeroSum(E)
sage: Z.weight()
2

zerosum(Delta=1, tau=0, function='sincsquared_fast', ncpus=None)#

Bound from above the analytic rank of the form attached to self.

This bound is obtained by computing \(\sum_{\gamma} f(\Delta(\gamma-\tau))\), where \(\gamma\) ranges over the imaginary parts of the zeros of \(L_E(s)\) along the critical strip, and \(f(x)\) is an appropriate even continuous \(L_2\) function such that \(f(0)=1\).

If \(\tau=0\), then as \(\Delta\) increases this sum converges from above to the analytic rank of the \(L\)-function, as \(f(0) = 1\) is counted with multiplicity \(r\), and the other terms all go to 0 uniformly.

INPUT:

Delta – positive real number (default: 1) parameter denoting the tightness of the zero sum.
tau – real parameter (default: 0) denoting the offset of the sum to be computed. When \(\tau=0\) the sum will converge to the analytic rank of the \(L\)-function as \(\Delta\) is increased. If \(\tau\) is the value of the imaginary part of a noncentral zero, the limit will be 1 (assuming the zero is simple); otherwise, the limit will be 0. Currently only implemented for the sincsquared and cauchy functions; otherwise ignored.
function – string (default: “sincsquared_fast”) - the function \(f(x)\) as described above. Currently implemented options for \(f\) are
- sincsquared – \(f(x) = \left(\frac{\sin(\pi x)}{\pi x}\right)^2\)
- gaussian – \(f(x) = e^{-x^2}\)
- sincsquared_fast – Same as “sincsquared”, but implementation optimized for elliptic curve \(L\)-functions, and tau must be 0. self must be attached to an elliptic curve over \(\QQ\) given by its global minimal model, otherwise the returned result will be incorrect.
- sincsquared_parallel – Same as “sincsquared_fast”, but optimized for parallel computation with large (>2.0) \(\Delta\) values. self must be attached to an elliptic curve over \(\QQ\) given by its global minimal model, otherwise the returned result will be incorrect.
- cauchy – \(f(x) = \frac{1}{1+x^2}\); this is only computable to low precision, and only when \(\Delta < 2\).
ncpus – (default: None) If not None, a positive integer defining the number of CPUs to be used for the computation. If left as None, the maximum available number of CPUs will be used. Only implemented for algorithm=”sincsquared_parallel”; ignored otherwise.

Warning

Computation time is exponential in \(\Delta\), roughly doubling for every increase of 0.1 thereof. Using \(\Delta=1\) will yield a computation time of a few milliseconds; \(\Delta=2\) takes a few seconds, and \(\Delta=3\) takes upwards of an hour. Increase at your own risk beyond this!

OUTPUT:

A positive real number that bounds from above the number of zeros with imaginary part equal to \(\tau\). When \(\tau=0\) this is an upper bound for the \(L\)-function’s analytic rank.

See also

analytic_rank_bound() for more documentation and examples on calling this method on elliptic curve \(L\)-functions.

EXAMPLES:

sage: E = EllipticCurve("389a"); E.rank()
2
sage: Z = LFunctionZeroSum(E)
sage: E.lseries().zeros(3)
[0.000000000, 0.000000000, 2.87609907]
sage: Z.zerosum(Delta=1,function="sincsquared_fast") # tol 1.0e-13
2.037500084595065
sage: Z.zerosum(Delta=1,function="sincsquared_parallel") # tol 1.0e-11
2.037500084595065
sage: Z.zerosum(Delta=1,function="sincsquared") # tol 1.0e-13
2.0375000845950644
sage: Z.zerosum(Delta=1,tau=2.876,function="sincsquared") # tol 1.0e-13
1.075551295651154
sage: Z.zerosum(Delta=1,tau=1.2,function="sincsquared") # tol 1.0e-13
0.10831555377490683
sage: Z.zerosum(Delta=1,function="gaussian") # tol 1.0e-13
2.056890425029435