Function: lfundual
Section: l_functions
C-Name: lfundual
Prototype: Gb
Help: lfundual(L): creates the Ldata structure (without
 initialization) corresponding to the dual L-function of L.
Doc: creates the \kbd{Ldata} structure (without initialization) corresponding
 to the dual L-function $\hat{L}$ of $L$. If $k$ and $\varepsilon$ are
 respectively the weight and root number of $L$, then the following formula
 holds outside poles, up to numerical errors:
 $$\Lambda(L, s) = \varepsilon \Lambda(\hat{L}, k - s).$$

 \bprog
 ? L = lfunqf([1,2,3,4]); [N,k] = lfunparams(L);
 ? M = lfundual(L); lfuncheckfeq(M)
 %2 = -128
 ? [r,R,w] = lfunrootres(L);
 ? check(s) = exponent(lfunlambda(L, s) - w * lfunlambda(M, k-s));
 ? check(1 + I*Pi)
 %5 = -130
 @eprog
