I recommend generatingfunctionology by herbert wilf for the interested. The beautiful idea here is to consider formal power series (encoding sequences of value

I recommend generatingfunctionology by herbert wilf for the interested. The beautiful idea here is to consider formal power series (encoding sequences of values in polynomials without demanding convergence) https://en.wikipedia.org/wiki/Formal_power_series https://en.wikipedia.org/wiki/Formal_power_series