In [[coding 1}}, so the expression simplifies to:
Λ'(x) is the formal derivative of the error locator polynomial Λ(x):
In the above expression, note that i is an integer, and λi would be an element of the finite field. The operator · represents ordinary multiplication (repeated addition in the finite field) and not the finite field's multiplication operator.
Lagrange interpolation gives a derivation of the Forney algorithm.
Define the erasure locator polynomial
Where the erasure locations are given by ji. Apply the procedure described above, substituting Γ for Λ.
If both errors and erasures are present, use the error-and-erasure locator polynomial