// lgamma(1-z) + lgamma(z) = log(pi) - log(sin(pi*z))
// sign of gamma(1-z) = sign of gamma(z) * sign of sin(pi*z)
v_z = sin(M_PI * x);
// lgamma(1-z) + lgamma(z) = log(pi) - log(sin(pi*z))
// sign of gamma(1-z) = sign of gamma(z) * sign of sin(pi*z)
v_z = sin(M_PI * x);