lfuncost

Function: lfuncost
Section: l_functions
C-Name: lfuncost0
Prototype: GDGD0,L,b
Help: lfuncost(L,{sdom},{der=0}): estimate the cost of running
 lfuninit(L,sdom,der) at current bit precision. Returns [t,b], to indicate
 that t coefficients a_n will be computed at bit accuracy b. Subsequent
 evaluation of lfun at s evaluates a polynomial of degree t at exp(h s).
 If L is already an Linit, then sdom and der are ignored.
Doc: estimate the cost of running
 \kbd{lfuninit(L,sdom,der)} at current bit precision. Returns $[t,b]$, to
 indicate that $t$ coefficients $a_n$ will be computed, as well as $t$ values of
 \tet{gammamellininv}, all at bit accuracy $b$.
 A subsequent call to \kbd{lfun} at $s$ evaluates a polynomial of degree $t$
 at $\exp(h s)$ for some real parameter $h$, at the same bit accuracy $b$.
 If $L$ is already an \kbd{Linit}, then \var{sdom} and \var{der} are ignored
 and are best left omitted; the bit accuracy is also inferred from $L$: in
 short we get an estimate of the cost of using that particular \kbd{Linit}.

 \bprog
 ? \pb 128
 ? lfuncost(1, [100]) \\ for zeta(1/2+I*t), |t| < 100
 %1 = [7, 242]  \\ 7 coefficients, 242 bits
 ? lfuncost(1, [1/2, 100]) \\ for zeta(s) in the critical strip, |Im s| < 100
 %2 = [7, 246]  \\ now 246 bits
 ? lfuncost(1, [100], 10) \\ for zeta(1/2+I*t), |t| < 100
 %3 = [8, 263]  \\ 10th derivative increases the cost by a small amount
 ? lfuncost(1, [10^5])
 %3 = [158, 113438]  \\ larger imaginary part: huge accuracy increase

 ? L = lfuncreate(polcyclo(5)); \\ Dedekind zeta for Q(zeta_5)
 ? lfuncost(L, [100]) \\ at s = 1/2+I*t), |t| < 100
 %5 = [11457, 582]
 ? lfuncost(L, [200]) \\ twice higher
 %6 = [36294, 1035]
 ? lfuncost(L, [10^4])  \\ much higher: very costly !
 %7 = [70256473, 45452]
 ? \pb 256
 ? lfuncost(L, [100]); \\ doubling bit accuracy
 %8 = [17080, 710]
 @eprog\noindent In fact, some $L$ functions can be factorized algebraically
 by the \kbd{lfuninit} call, e.g. the Dedekind zeta function of abelian
 fields, leading to much faster evaluations than the above upper bounds.
 In that case, the function returns a vector of costs as above for each
 individual function in the product actually evaluated:
 \bprog
 ? L = lfuncreate(polcyclo(5)); \\ Dedekind zeta for Q(zeta_5)
 ? lfuncost(L, [100])  \\ a priori cost
 %2 = [11457, 582]
 ? L = lfuninit(L, [100]); \\ actually perform all initializations
 ? lfuncost(L)
 %4 = [[16, 242], [16, 242], [7, 242]]
 @eprog\noindent The Dedekind function of this abelian quartic field
 is the product of four Dirichlet $L$-functions attached to the trivial
 character, a nontrivial real character and two complex conjugate
 characters. The nontrivial characters happen to have the same conductor
 (hence same evaluation costs), and correspond to two evaluations only
 since the two conjugate characters are evaluated simultaneously.
 For a total of three $L$-functions evaluations, which explains the three
 components above. Note that the actual cost is much lower than the a priori
 cost in this case.
Variant: Also available is
 \fun{GEN}{lfuncost}{GEN L, GEN dom, long der, long bitprec}
 when $L$ is \emph{not} an \kbd{Linit}; the return value is a \typ{VECSMALL}
 in this case.