Function: thetanull
Section: transcendental
C-Name: thetanull
Prototype: GDGp
Help: thetanull(tau,{flag=0}): Jacobi Thetanullwerte: if flag is not 1 or
 [1,1], same as theta(0, tau, flag). Otherwise, (d/dz)theta(z,tau,flag) at z=0.
 If flag = 0 or omitted, vector of all four.
Doc: Jacobi Thetanullwerte: if \kbd{flag} is present, but neither $0$, $1$
 nor \kbd{[1,1]},
 this is the same as \kbd{theta(0,tau,flag)}. Note that $\theta_{1,1}(0,\tau)
 = \theta_1(0,\tau) = 0$ for all $\tau$. So we compute the derivative of
 \kbd{theta(z, tau, flag)} with respect to $z$ at $z=0$ for $\fl = 1$
 or \kbd{[1,1]}.
 Finally, if $\fl = 0$ or omitted, returns the vector of all four values
 $$[\theta_{0,0}(0,\tau), \theta_{0,1}(0,\tau), \theta_{1,0}(0,\tau),
 \theta'_{1,1}(0,\tau)]\;.$$
 (Note the derivative in the last entry!)
