How to graph series in Desmos $f\left(x\right)=\sum_{n=1}^{x}nb-f_{1}\left(f\left(x-1\right)\right)$

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$$f\left(x\right)=\sum_{n=1}^{x}nb-f_{1}\left(f\left(n-1\right)\right)$$

$$b=2$$

$$f_1(x)=x^2$$

$$f(1)=b$$

How can I define this whole series in Desmos?