my question is when I see the answer for this recursion by substitution method, I see letter "d" is used which is not in the textbook "Introduction to Algorithms"

so, why we use "d" in the solution. could you give me a reference to provide your answer if possible

thank you

1.2.2 Back to the substitution method Now we can check our guess using the substitution method. Recall that the original recur- rence was T(n) = 3T([n/4]) + (ny). We want to show that T(n) 0. By the induction hypothesis, we have that T(n/40) 0 as before, we have T(n) (16/13)c)