fn exponential_kernel<const N: usize>(v: Jet<N>) -> Jet<N>
F_1(v) = Gamma(1, v) / (-v) = exp(-v) / (-v) of the 3D spherical and 2D cylindrical reciprocal sums, in closed form with dF_1/dv = F_2 = (1 + v) exp(-v) / v^2.
F_1(v) = Gamma(1, v) / (-v) = exp(-v) / (-v)
dF_1/dv = F_2 = (1 + v) exp(-v) / v^2