Math Problem Statement
Use generating functions to find an explicit formula for the Fibonacci numbers.
Solution
To find an explicit formula for the Fibonacci numbers using generating functions, follow these steps:
Step 1: Define the Fibonacci Sequence
The Fibonacci sequence is defined by the recurrence relation: with initial conditions:
Step 2: Define the Generating Function
Let be the generating function for the Fibonacci sequence:
Step 3: Express the Recurrence Relation in Terms of
Using the recurrence relation , we have: Substitute the recurrence relation into the sum: This can be split into two sums: Re-index the sums: This simplifies to: Thus, we can express as: Since , this reduces to:
Step 4: Solve for
Now, collect terms involving : Factor out : So, we have:
Step 5: Find the Explicit Formula
The generating function can be expanded using partial fractions. First, find the roots of the denominator . The roots are: So, we can express as: Using the geometric series expansion , we get: Thus: Comparing this with the original generating function , we have:
Conclusion
The explicit formula for the Fibonacci numbers is:
This is known as Binet's Formula.
Would you like more details or have any questions?
Here are five questions to consider:
- How is the generating function related to the Fibonacci sequence?
- What is the significance of the roots and in the formula?
- How can partial fractions be used to solve generating functions?
- What are the applications of Binet's formula in combinatorics?
- How would the generating function approach change if the Fibonacci sequence had different initial conditions?
Tip: Understanding generating functions can simplify solving complex recurrence relations in sequences.
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Math Problem Analysis
Mathematical Concepts
Generating Functions
Fibonacci Sequence
Recurrence Relations
Geometric Series
Formulas
F_n = F_{n-1} + F_{n-2}
G(x) = \sum_{n=0}^{\infty} F_n x^n
G(x) = \frac{x}{1 - x - x^2}
F_n = \frac{1}{\sqrt{5}} \left(\alpha^n - \beta^n\right)
Theorems
Binet's Formula
Geometric Series Expansion
Suitable Grade Level
College or Advanced High School (Grades 11-12)