as to be the difference between

[capcap23]

Let’s now define a new function, g(x), as to be the difference between f(x) and the equation of the secant line or, ... Source

Is use of "as to" correct here? If so, could you explain the grammar behind this.
As far as I know, "as to" is used with "such" or "so".

 

[Glizdka]

It looks like the natural part that follows the verb define. We define one thing as another.

"Let's define g(x) as the difference between f(x) and the equation of the secant line."

I'm not sure about to be in here, but it looks optional.

Does that help?

 

[SoothingDave]

I think you could define it "as" or define it "to be." But not both combined.