Data Structures and Algorithms
with Object-Oriented Design Patterns in C++ 
In this section we present a technique for solving a recurrence
relation such as Equation 
called
repeated substitution .
The basic idea is this:
Given that 
,
then we may also write 
, provided n>1.
Since T(n-1) appears in the right-hand side of the former equation,
we can substitute for it the entire right-hand side of the latter.
By repeating this process we get
The next step takes a little intuition: We must try to discern the pattern which is emerging. In this case it is obvious:
where 
.
Of course, if we have doubts about our intuition,
we can always check our result by induction:
Base Case
Clearly the formula is correct for k=1,
since 
.
Inductive Hypothesis
Assume that 
for 
.
By this assumption
Note also that using the original recurrence relation we can write
for 
.
Substituting Equation
in the right-hand side of Equation gives
Therefore, by induction on l, our formula is correct
for all 
.
So, we have shown that , for .
Now, if n was known,
we would repeat the process of substitution until we got T(0)
on the right hand side.
The fact that n is unknown should not deter us--we get T(0) on the right hand side when n-k=0.
I.e., k=n.
Letting k=n we get
where 
and
.
Copyright © 1997 by Bruno R. Preiss, P.Eng. All rights reserved.
