Corollary 15.29.
Pages 2

Use the formula (2.15) on each term of the sum and Corollary 15.29 for ∆S. Take the first term outside of the first sum. Apply the induction assumption for p − 1 on the second sum. Change the summation index by 1 in the last sum. Then add the first and third expressions with the second and last sums cancelling.