Function: lfunqf
Section: l_functions
C-Name: lfunqf
Prototype: Gp
Help: lfunqf(Q): returns the Ldata structure attached to the
 theta function of the lattice attached to the definite positive quadratic
 form Q.
Doc: returns the \kbd{Ldata} structure attached to the $\Theta$ function
 of the lattice attached to the primitive form proportional to the definite
 positive quadratic form $Q$.

 \bprog
 ? L = lfunqf(matid(2));
 ? lfunqf(L,2)
 %2 = 6.0268120396919401235462601927282855839
 ? lfun(x^2+1,2)*4
 %3 = 6.0268120396919401235462601927282855839
 @eprog\noindent A vector is interpreted as a diagonal matrix.

 The following computes the Madelung constant:
 \bprog
 ? L1 = lfunqf([1,1,1]);
 ? L2 = lfunqf([4,1,1]);
 ? L3 = lfunqf([4,4,1]);
 ? F(s) = 6*lfun(L2,s) - 12*lfun(L3,s) - lfun(L1,s)*(1-2^(3-2*s));
 ? F(1/2)
 %5 = -1.7475645946331821906362120355443974034
 @eprog
