"S₀ = I : Chebyshev() → Ultraspherical(1)" :UndefVarError: I not defined

[Jiaqi-knight]

In lecture16, here are several codes:
S₀ = I : Chebyshev() → Ultraspherical(1)
f = Fun(exp, Chebyshev())
g = S₀*f
g(4) - exp(4)

I found the error when I run it:
UndefVarError: I not defined

Stacktrace:
[1] top-level scope at In[84]:1

Then I search for solutions, referred in
"JuliaApproximation/ApproxFun.jl#590 (comment)"
it can change to code "Conversion(Chebyshev(), Ultraspherical(1))" instead, and it works.

However, the final result such like "g(3) - exp(3)" is equal to -20.085536923187668, not approach to zero.

[dlfivefifty]

Try adding using LinearAlgebra

[Jiaqi-knight]

yes, it works, but the example seems not good?

[dlfivefifty]

Because you can only evaluate a function inside its domain of definition, in this case the default -1 ..1

[Jiaqi-knight]

I got it!!!