not[idx_] := Complement[Range[NN], idx]
SC[A_, idx_] := A[[idx, idx]] - A[[idx, not@idx]] . Inverse[A[[not@idx, not@idx]]] . A[[not@idx, idx]]
I am not sure how to interpret the two lines of code
We have matrix $A$ and $SC$
but how are they getting SC from A using the code I dont understand