How do I get the sum of such a sequence:
$$1 + x^{-1} + x^{-3} + x^{-6} + \dotsb,$$
where the exponents are actually sum of arithmetic progression? I.e.,
$$x^{-0} + x^{-(0 + 1)} + x^{-(0 + 1 + 2)} + x^{-(0 + 1 + 2 + 3)} + \dotsb,$$
which can also be expressed as
$$\sum_{i=0}^{\infty} x^{-\frac{i(i + 1)}{2}}.$$