SUM OF PRODUCT OF RECIPROCALS OF FIBONACCI NUMBERS
A Thesis Submitted in the Partial Fulfillment of the Requirements of Degree for
Integrated M.Sc.
In Mathematics
Submitted by
Kappagantu Prudhavi Nag Roll Number: 410MA5016
Under the Guidance of Professor G. K. Panda Department of Mathematics
National Institute of Technology, Rourkela
May 2015
Page | 2
CERTIFICATE
Dr. Gopal Krishna Panda
Professor of Mathematics May 11, 2015
This is to certify that the project report with title โSUM OF PRODUCT OF RECIPROCALS OF FIBONACCI NUMBERSโ submitted by Mr. Kappagantu Prudhavi Nag, Roll No. 410MA5016, to the National Institute of Technology, Rourkela, Odisha for the partial fulfillment of the requirements of Integrated M.Sc. degree in Mathematics, is a bonafide research work carried out by him under my supervision and guidance. The content of this report in full or part has not been submitted to any other Institute or University for the award of any degree or diploma.
Gopal Krishna Panda
Page | 3
ACKNOWLEDGEMENTS
I would like to express my special appreciation and thanks to my supervisor Professor G. K. Panda who has been a great mentor for me. I thank him for his suggestion for providing a beautiful problem in number theory.
I am grateful to Prof. Sunil Kumar Sarangi, Director, National Institute of Technology, Rourkela for providing excellent facilities in the Institute for carrying out research.
I am thankful to the Head, mathematics and professors of department for their valuable help during the preparation of this work.
There are no words to explain how grateful I am to my parents for all of their sacrifices and prayers that they made for me. I also like to thank my brother who helped me in this research and also gave a few ideas which helped me in successful completion of this project.
I also take this opportunity to thank all of my friends who helped me during the preparation of this work.
Place: Rourkela
Date: Kappagantu Prudhavi Nag
Department of Mathematics
National Institute of Technology Rourkela
Page | 4
ABSTRACT
Fibonacci numbers are the number sequences which follow the linear mathematical recurrence๐น0 = 0, ๐น1 = 1 and ๐น๐ = ๐น๐โ1+ ๐น๐โ2 ๐ โฅ 2. In this work, we study certain sum formulas involving products of reciprocals of Fibonacci numbers. Sum formulas with alternating signs are also studied.
Page | 5
TABLE OF CONTENTS
1. Notations ...6
2. Introduction ...7
i. Mathematics of Fibonacci Numbers ... 7
ii. Fibonacci Numbers with Negative Indices ... 8
3. Sum of Reciprocals of Fibonacci Numbers with Positive Indices ... 11
i. Order 2 ... 11
a. Non-Alternating Sum... 11
b. Alternating Sum ... 13
ii. Sum With Indices in A.P. ... 16
a. Non-Alternating Sum of Order 1 ... 16
b. Alternating Sum of Order 1 ... 16
4. Sum of Reciprocals of Fibonacci Numbers with Negative Indices ... 19
i. Order 1 ... 19
a. Non-Alternating Sum... 20
b. Alternating Sum ... 20
ii. Order 2 ... 20
a. Non-Alternating Sum... 21
b. Alternating Sum ... 21
iii. Sum with Indices in A.P. ... 22
a. Non-Alternating Sum of Order 1 ... 23
b. Alternating Sum of Order 1 ... 24
5. References ... 26
Page | 6
NOTATIONS
The following notations will be frequently used in this thesis.
๏ท ๐ฝ๐ = โ 1
๐น๐ ๐๐=1
๏ท ๐พ๐ = โ (โ1)๐
๐น๐ ๐๐=1
๏ท ๐ฝ๐(๐) = โ 1
๐น๐+๐ ๐๐=1
๏ท ๐พ๐(๐) = โ (โ1)๐
๐น๐+๐ ๐๐=1
๏ท ๐ฝ๐โ = โ 1
๐นโ๐ ๐๐=1
๏ท ๐พ๐โ = โ (โ1)๐
๐นโ๐ ๐๐=1
๏ท โ๐ = โ 1
๐นโ๐ ๐๐=1
๏ท ๐๐= โ (โ1)๐
๐นโ๐ ๐๐=1
๏ท โ๐(๐) = โ 1
๐นโ๐โ๐ ๐๐=1
๏ท ๐๐(๐) = โ 1
๐นโ๐โ๐ ๐๐=1
๏ท โ๐(0, ๐) = โ 1
๐นโ๐๐นโ๐โ๐ ๐๐=1
๏ท ๐๐(0, ๐) = โ (โ1)๐
๐นโ๐๐นโ๐โ๐ ๐๐=1
๏ท โ๐(๐, ๐) = โ 1
๐นโ๐โ๐๐นโ๐โ๐ ๐๐=1
๏ท ๐๐(๐, ๐) = โ (โ1)๐
๐นโ๐โ๐๐นโ๐โ๐ ๐๐=1
๏ท โ๐โ = โ 1
๐นโโ๐ ๐๐=1
๏ท ๐๐โ = โ (โ1)๐
๐นโโ๐ ๐๐=1
๏ท โ๐โ(๐) = โ 1
๐นโโ๐โ๐ ๐๐=1
๏ท ๐๐โ(๐) = โ (โ1)๐
๐นโโ๐โ๐ ๐๐=1
Page | 7
CHAPTER 1 INTRODUCTION
Fibonacci sequence of numbers is one of the most intriguing number sequence in mathematics. The series is named after the famous Italian Mathematician Fibonacci of the Bonacci family. He is also known as the Leonardo of Pisa. The following problem proposed by Fibonacci himself gave birth to the sequence.
The Fibonacci Problem:
Suppose there are two newborn rabbits, one male and one female.Find the number of rabbits produced in a year if [1]
๏ท Each pair takes one month to become mature
๏ท Each pair produces a mixed pair every month from the second month
๏ท All rabbits are immortal
Solution:
For convenience let us assume that the rabbits are born on January 1st and we need to find the number of rabbits on December 1st. The table below is used to find the solution of the problem.From the above table it is evident that the number of rabbits at the end of the year are 144. If observed closely it is observed that the new number is equal to the sum of the previous two numbers.
MATHEMATICS OF FIBONACCI NUMBERS
The numbers in the bottom row are called the Fibonacci numbers. From the table a recursive relation is yielded as below
๐น๐ = ๐น๐โ1+ ๐น๐โ2 , ๐ โฅ 2.
where ๐น0 = 0 and ๐น1 = 1 [2]. Sometimes it is customary to start the Fibonacci numbers from ๐น1 instead of ๐น0. Then the initial two conditions become ๐น1 = 1 and ๐น2 = 1. With any of the above two conditions the series generated is the same.
No. of Pairs Jan Feb Mar April May June July Aug Sep Oct Nov Dec
Adults 0 1 1 2 3 5 8 13 21 34 55 89
Babies 1 0 1 1 2 3 5 8 13 21 34 34
Total 1 1 2 3 5 8 13 21 34 55 89 144
Page | 8 It is surprising that Fibonacci numbers can be extracted from Pascalโs triangle. The above observation was confirmed by Lucas in 1876 when he derived a straightforward formula to find the Fibonacci numbers [1]
๐น๐ = โ (๐ โ ๐ โ 1
๐ )
โ(๐โ1) 2โ โ
๐=0
, ๐ โฅ 1.
In 1843 a French mathematician named Jacques Philippe Marie Binet invented a way to calculate the ๐๐กโ Fibonacci numbers. If ๐ =1+โ5
2 and ๐ =1โโ5
2 then ๐น๐ = ๐๐โ๐๐
โ5 [3].
The above formula shows an interesting aspect that the Fibonacci number can be written in terms of the golden ratio. Fibonacci numbers appear in many places in both nature and mathematics. They occur in music, geography, nature, and geometry. They can be found in the spiral arrangements of seeds of sunflowers, the scale patterns of pine cones, the arrangement of leaves and the number of petals on the flower.
FIBONACCI NUMBERS WITH NEGATIVE INDICES
The negative subscript of the Fibonacci numbers can be converted to positive subscript as indicated in [4]. The recurrence relation for the negative Fibonacci numbers is as follows:
๐นโ๐ = ๐น2โ๐โ ๐น1โ๐, ๐ โฅ 2
The initial conditions for these numbers are ๐น0 = 0 and ๐น1 = 1. It is observed that the negative Fibonacci numbers have the same initial conditions as of the positive Fibonacci numbers.
Rabinowitz [5] stated that the alternating general summation of order 1 is given by
๐พ๐(๐) = โ(โ1)๐ ๐น๐+๐
๐
๐=1
= ๐พ๐+๐ โ ๐พ๐.
We disprove the above identity using a counter example.
Now let us take ๐ = 3 and ๐ = 5 and find the value of ๐พ๐(๐).
โ(โ1)๐
๐น๐+3
5
๐=1
=โ1
๐น4 + 1 ๐น5+โ1
๐น6 + 1 ๐น7+โ1
๐น8 ๐พ8 =โ1
๐น1 + 1 ๐น2+โ1
๐น3 + 1 ๐น4+โ1
๐น5 + 1 ๐น6+โ1
๐น7 + 1 ๐น8
Page | 9 ๐พ3 =โ1
๐น1 + 1 ๐น2+โ1
๐น3 ๐พ๐+๐โ ๐พ๐= 1
๐น4+โ1 ๐น5 + 1
๐น6+โ1 ๐น7 + 1
๐น8โ ๐พ๐(๐).
It can be checked that the identity for ๐พ๐(๐) is wrong when ๐ is odd and correct when ๐ is even.
Claim:
If ๐ > 0 then ๐พ๐(๐) = {๐พ๐โ ๐พ๐+๐ if ๐ is odd, ๐พ๐+๐โ ๐พ๐ if ๐ is even.Proof:
๐พ๐+๐โ ๐พ๐ = โ(โ1)๐ ๐น๐
๐+๐
๐=1
โ โ(โ1)๐ ๐น๐
๐
๐=1
= โ (โ1)๐ ๐น๐
๐+๐
๐=๐+1
.
Case 1. ๐ is odd, ๐ is odd
๐พ๐(๐) = โ(โ1)๐ ๐น๐+๐
๐
๐=1
= โ1 ๐น1+๐+ 1
๐น2+๐+ โฏ + 1
๐น(๐โ1)+๐+ โ1 ๐น๐+๐. ๐พ๐+๐ โ ๐พ๐ = โ (โ1)๐
๐น๐
๐+๐
๐=๐+1
= 1
๐น1+๐ + โ1
๐น2+๐+ โฏ + โ1
๐น(๐โ1)+๐+ 1 ๐น๐+๐
= โ ( โ1 ๐น1+๐+ 1
๐น2+๐+ โฏ + 1
๐น(๐โ1)+๐ + โ1 ๐น๐+๐)
= โ๐พ๐(๐).
Case 2. ๐ is odd, ๐ is even
๐พ๐(๐) = โ(โ1)๐ ๐น๐+๐
๐
๐=1
= โ1 ๐น1+๐+ 1
๐น2+๐+ โฏ + โ1
๐น(๐โ1)+๐+ 1 ๐น๐+๐.
๐พ๐+๐ โ ๐พ๐= โ (โ1)๐ ๐น๐
๐+๐
๐=๐+1
= 1
๐น1+๐+ โ1
๐น2+๐+ โฏ + 1
๐น(๐โ1)+๐+ โ1 ๐น๐+๐
= โ ( โ1 ๐น1+๐+ 1
๐น2+๐+ โฏ + โ1
๐น(๐โ1)+๐+ 1 ๐น๐+๐)
= โ๐พ๐(๐).
Page | 10 Case 3. ๐ is even , ๐ is odd
๐พ๐(๐) = โ(โ1)๐ ๐น๐+๐
๐
๐=1
= โ1 ๐น1+๐+ 1
๐น2+๐+ โฏ + 1
๐น(๐โ1)+๐+ โ1 ๐น๐+๐. ๐พ๐+๐โ ๐พ๐ = โ (โ1)๐
๐น๐
๐+๐
๐=๐+1
= โ1 ๐น1+๐+ 1
๐น2+๐+ โฏ + 1
๐น(๐โ1)+๐+ โ1 ๐น๐+๐
= ๐พ๐(๐).
Case 4. ๐ is even , ๐ is even
๐พ๐(๐) = โ(โ1)๐ ๐น๐+๐
๐
๐=1
= โ1 ๐น1+๐+ 1
๐น2+๐+ โฏ + โ1
๐น(๐โ1)+๐+ 1 ๐น๐+๐. ๐พ๐+๐โ ๐พ๐ = โ (โ1)๐
๐น๐
๐+๐
๐=๐+1
= โ1 ๐น1+๐+ 1
๐น2+๐+ โฏ + โ1
๐น(๐โ1)+๐+ 1 ๐น๐+๐
= ๐พ๐(๐).
โ
Page | 11
CHAPTER 2
SUM OF RECIPROCALS OF FIBONACCI NUMBERS WITH POSITIVE INDICES
The following notations for the alternating and non-alternating sum of ๐๐กโ order are available in[5].
๐(๐1, ๐2, โฆ , ๐๐โ1, ๐๐ ) = โ 1 ๐น๐+๐1๐น๐+๐2โฆ๐น
๐+๐๐ ๐
๐=1
๐(๐1, ๐2, โฆ , ๐๐โ1, ๐๐ ) = โ (โ1)๐ ๐น๐+๐1๐น๐+๐2โฆ๐น
๐+๐๐ ๐
๐=1
The sum ๐ is called the non-alternating summation of order ๐. The second sum ๐ is called the alternating sum of order ๐. In both the cases 0 < ๐1 < ๐2 < โฏ < ๐๐โ1 < ๐๐.
ORDER 2
We consider first the problem of finding the following second order sums.
๐ฝ๐(๐, ๐ ) = โ 1 ๐น๐+๐๐น๐+๐
๐
๐=1
๐พ๐(๐, ๐ ) = โ (โ1)๐ ๐น๐+๐๐น๐+๐
๐
๐=1
NON-ALTERNATING SUM
For ๐ > 0, Rabinowitz[5] got the following formula.
๐ฝ๐(0, ๐) = โ 1 ๐น๐๐น๐+๐ =
{ 1
๐น๐ โ ( 1
๐น๐+2๐๐น๐+2๐+1โ 1 ๐น2๐๐น2๐+1)
โ๐ 2โ โ
๐=1
+๐๐
๐น๐ if ๐ is odd, 1
๐น๐โ ( 1
๐น2๐โ1๐น2๐ โ 1
๐น๐+2๐โ1๐น๐+2๐)
๐ 2โ
๐=1
if ๐ is even.
๐
๐=1
where ๐๐= โ 1
๐น๐๐น๐+1
๐๐=1 . The above sum is called the non-alternating sum of order 2. The aim is to find an equivalent expression of ๐ฝ๐(๐, ๐). To do this we use the help of the following result.
Page | 12 ๐โ๐๐๐๐๐ 1: ๐น๐๐ 0 < ๐ < ๐ ๐กโ๐๐ ๐น๐ (๐, ๐) = ๐น๐+๐ (0, ๐ โ ๐) โ ๐น๐ (0, ๐ โ ๐).
๐๐๐๐๐:We start from the right hand side of the equation and come to the left hand side.
๐ฝ๐+๐(0, ๐ โ ๐) โ ๐ฝ๐(0, ๐ โ ๐) = โ 1
๐น๐๐น๐+๐โ๐โ โ 1 ๐น๐๐น๐+๐โ๐
๐
๐=1 ๐+๐
๐=1
= โ 1
๐น๐๐น๐+๐โ๐
๐+๐
๐=๐+1
= 1
๐น๐+1๐น๐+1+ 1
๐น๐+2๐น๐+2+ โฏ + 1
๐น๐+๐โ1๐น๐+๐โ1+ 1 ๐น๐+๐๐น๐+๐
= โ 1
๐น๐+๐๐น๐+๐
๐
๐=1
= ๐ฝ๐(๐, ๐)
โ By using the above theorem and the formula for ๐ฝ๐(0, ๐) stated by Rabinowitz [5] the expression for ๐ฝ๐(๐, ๐) is calculated.
๐โ๐๐๐๐๐ 2: ๐ผ๐ 0 < ๐ < ๐ ๐กโ๐๐
๐ฝ๐(๐, ๐) = { 1
๐น๐โ๐ โ ( 1
๐น๐+๐+2๐๐น๐+๐+2๐+1โ 1 ๐น๐+2๐๐น๐+2๐+1)
โ(๐โ๐) 2โ โ
๐=1
+๐ฝ๐(๐, ๐ + 1)
๐น๐โ๐ if (๐ โ ๐)is odd,
1
๐น๐โ๐ โ ( 1
๐น๐+2๐โ1๐น๐+2๐โ 1
๐น๐+๐+2๐โ1๐น๐+๐+2๐)
(๐โ๐) 2โ
๐=1
if (๐ โ ๐)is even.
๐๐๐๐๐
:
We take the help of Theorem 1 to prove this theorem.๐ฝ๐+๐(0, ๐ โ ๐) =
{ 1
๐น๐โ๐ โ ( 1
๐น๐+๐+2๐๐น๐+๐+2๐+1โ 1 ๐น2๐๐น2๐+1)
โ(๐โ๐) 2โ โ
๐=1
+๐๐+๐
๐น๐โ๐ if (๐ โ ๐) is odd, 1
๐น๐โ๐ โ ( 1
๐น2๐โ1๐น2๐โ 1
๐น๐+๐+2๐โ1๐น๐+๐+2๐)
๐(๐โ๐) 2โ โ๐
โ2
๐=1
if (๐ โ ๐) is even.
๐ฝ๐(0, ๐ โ ๐) = { 1
๐น๐โ๐ โ ( 1
๐น๐+2๐๐น๐+2๐+1โ 1 ๐น2๐๐น2๐+1)
โ(๐โ๐) 2โ โ
๐=1
+ ๐๐
๐น๐โ๐ if (๐ โ ๐) is odd, 1
๐น๐โ๐ โ ( 1
๐น2๐โ1๐น2๐โ 1
๐น๐+2๐โ1๐น๐+2๐)
(๐โ๐) 2โ
๐=1
if (๐ โ ๐) is even.
Depending upon the parity of (๐ โ ๐), two different cases are taken into consideration
Page | 13 Case 1. (๐ โ ๐) ๐ข๐ฌ ๐จ๐๐
In this case,
๐ฝ๐(๐, ๐) = 1
๐น๐โ๐ โ ( 1
๐น๐+๐+2๐๐น๐+๐+2๐+1โ 1
๐น2๐๐น2๐+1โ 1
๐น๐+2๐๐น๐+2๐+1+ 1 ๐น2๐๐น2๐+1)
โ(๐โ๐) 2โ โ
๐=1
+๐๐+๐โ ๐๐ ๐น๐โ๐
= 1
๐น๐โ๐ โ ( 1
๐น๐+๐+2๐๐น๐+๐+2๐+1โ 1
๐น๐+2๐๐น๐+2๐+1) +๐๐+๐โ ๐๐ ๐น๐โ๐ .
โ(๐โ๐) 2โ โ
๐=1
Now,
๐๐+๐ โ ๐๐ = โ 1 ๐น๐๐น๐+1
๐+๐
๐=1
โ โ 1
๐น๐๐น๐+1
๐
๐=1
= โ 1
๐น๐๐น๐+1
๐+๐
๐=๐+1
= โ 1
๐น๐+๐๐น๐+๐+1= ๐ฝ๐(๐, ๐ + 1)
๐
๐=1
๐ฝ๐(๐, ๐) = 1
๐น๐โ๐ โ ( 1
๐น๐+๐+2๐๐น๐+๐+2๐+1โ 1 ๐น๐+2๐๐น๐+2๐+1)
โ(๐โ๐) 2โ โ
๐=1
+๐ฝ๐(๐, ๐ + 1) ๐น๐โ๐ . Case 2. (๐ โ ๐) is even
๐ฝ๐(๐, ๐) = 1
๐น๐โ๐ โ ( 1
๐น2๐โ1๐น2๐โ 1
๐น๐+๐+2๐โ1๐น๐+๐+2๐โ 1
๐น2๐โ1๐น2๐โ 1
๐น๐+2๐โ1๐น๐+2๐)
(๐โ๐) 2โ
๐=1
= 1
๐น๐โ๐ โ ( 1
๐น๐+2๐โ1๐น๐+2๐โ 1
๐น๐+๐+2๐โ1๐น๐+๐+2๐)
(๐โ๐) 2โ
๐=1
.
โ
ALTERNATING SUM
Let ๐ > 0 . The following identity is available in from [6].
๐พ๐(0, ๐) = โ (โ1)๐ ๐น๐๐น๐+๐ = 1
๐น๐โ (๐น๐โ1
๐น๐ โ๐น๐+๐โ1 ๐น๐+๐ )
๐
๐=1 ๐
๐=1
.
The above is a sum with alternating sign (called an alternating sum) of order 2. We use the above sum to find a formula for ๐พ๐(๐, ๐). To achieve this, we use of the following result.
๐โ๐๐๐๐๐ 3: ๐ผ๐ 0 < ๐ < ๐ ๐กโ๐๐
๐พ๐(๐, ๐) = {๐พ๐(0, ๐ โ ๐) โ ๐พ๐+๐(0, ๐ โ ๐) if ๐ is odd, ๐พ๐+๐(0, ๐ โ ๐) โ ๐พ๐(0, ๐ โ ๐) if ๐ is even.
๐๐๐๐๐: Observe that
Page | 14 ๐พ๐+๐(0, ๐ โ ๐) โ ๐พ๐(0, ๐ โ ๐) = โ (โ1)๐
๐น๐๐น๐+๐โ๐
๐+๐
๐=1
โ โ (โ1)๐ ๐น๐๐น๐+๐โ๐
๐
๐=1
= โ (โ1)๐ ๐น๐๐น๐+๐โ๐.
๐+๐
๐=๐+1
We distinguish four cases:
Case 1. ๐ and ๐ are odd ๐พ๐(๐, ๐) = โ (โ1)๐
๐น๐+๐๐น๐+๐ = โ1
๐น๐+1๐น๐+1+ 1
๐น๐+2๐น๐+2+ โฏ + 1 ๐น๐+๐โ1๐น๐+๐โ1
๐
๐=1
+ โ1
๐น๐+๐๐น๐+๐.
๐พ๐+๐(0, ๐ โ ๐) โ ๐พ๐(0, ๐ โ ๐) = โ (โ1)๐ ๐น๐๐น๐+๐โ๐
๐+๐
๐=๐+1
= 1
๐น๐+1๐น๐+2+ โ1
๐น๐+2๐น๐+3+ โฏ + โ1
๐น๐+๐โ1๐น๐+๐โ1+ 1 ๐น๐+๐๐น๐+๐
= โ ( โ1
๐น๐+1๐น๐+1+ 1
๐น๐+2๐น๐+2+ โฏ + 1
๐น๐+๐โ1๐น๐+๐โ1+ โ1 ๐น๐+๐๐น๐+๐)
= โ๐พ๐(๐, ๐).
Case 2. ๐ is odd and ๐ is even ๐พ๐(๐, ๐) = โ (โ1)๐
๐น๐+๐๐น๐+๐ = โ1
๐น๐+1๐น๐+1+ 1
๐น๐+2๐น๐+2+ โฏ + โ1 ๐น๐+๐โ1๐น๐+๐โ1
๐
๐=1
+ 1
๐น๐+๐๐น๐+๐.
๐พ๐+๐(0, ๐ โ ๐) โ ๐พ๐(0, ๐ โ ๐) = โ (โ1)๐ ๐น๐๐น๐+๐โ๐
๐+๐
๐=๐+1
= 1
๐น๐+1๐น๐+2+ โ1
๐น๐+2๐น๐+3+ โฏ + 1
๐น๐+๐โ1๐น๐+๐โ1+ โ1 ๐น๐+๐๐น๐+๐
= โ ( โ1
๐น๐+1๐น๐+1+ 1
๐น๐+2๐น๐+2+ โฏ + โ1
๐น๐+๐โ1๐น๐+๐โ1++ 1 ๐น๐+๐๐น๐+๐)
= โ๐พ๐(๐, ๐).
Case 3. ๐ is even and ๐ is odd ๐พ๐(๐, ๐) = โ (โ1)๐
๐น๐+๐๐น๐+๐ = โ1
๐น๐+1๐น๐+1+ 1
๐น๐+2๐น๐+2+ โฏ + 1 ๐น๐+๐โ1๐น๐+๐โ1
๐
๐=1
+ โ1
๐น๐+๐๐น๐+๐.
๐พ๐+๐(0, ๐ โ ๐) โ ๐พ๐(0, ๐ โ ๐) = โ (โ1)๐ ๐น๐๐น๐+๐โ๐
๐+๐
๐=๐+1
= โ1
๐น๐+1๐น๐+1+ 1
๐น๐+2๐น๐+2+ โฏ + 1
๐น๐+๐โ1๐น๐+๐โ1++ โ1 ๐น๐+๐๐น๐+๐
Page | 15
= ๐พ๐(๐, ๐).
Case 4. ๐ and ๐ are even ๐พ๐(๐, ๐) = โ (โ1)๐
๐น๐+๐๐น๐+๐ = โ1
๐น๐+1๐น๐+1+ 1
๐น๐+2๐น๐+2+ โฏ + โ1 ๐น๐+๐โ1๐น๐+๐โ1
๐
๐=1
+ 1
๐น๐+๐๐น๐+๐
๐พ๐+๐(0, ๐ โ ๐) โ ๐พ๐(0, ๐ โ ๐) = โ (โ1)๐ ๐น๐๐น๐+๐โ๐
๐+๐
๐=๐+1
= โ1
๐น๐+1๐น๐+1+ 1
๐น๐+2๐น๐+2+ โฏ + โ1
๐น๐+๐โ1๐น๐+๐โ1++ 1 ๐น๐+๐๐น๐+๐
= ๐พ๐(๐, ๐).
โ The following theorem is one of the main results of this chapter.
๐โ๐๐๐๐๐ 4: ๐ผ๐ 0 < ๐ < ๐ ๐กโ๐๐
๐พ๐(๐, ๐) = {
1
๐น๐โ๐โ (๐น๐+๐+๐โ1
๐น๐+๐+๐ โ๐น๐+๐โ1 ๐น๐+๐ )
๐โ๐
๐=1
if ๐ is odd, 1
๐น๐โ๐โ (๐น๐+๐โ1
๐น๐+๐ โ๐น๐+๐+๐โ1 ๐น๐+๐+๐ )
๐โ๐
๐=1
if ๐ is even.
๐๐๐๐๐: We separate two cases:
Case 1. ๐ is odd
๐พ๐(0, ๐ โ ๐) โ ๐พ๐+๐(0, ๐ โ ๐) = 1
๐น๐โ๐โ ( ๐น๐โ1
๐น๐ โ๐น๐+๐โ1 ๐น๐+๐ )
๐โ๐
๐=1
โ 1
๐น๐โ๐โ ( ๐น๐โ1
๐น๐ โ๐น๐+๐+๐โ1 ๐น๐+๐+๐ )
๐โ๐
๐=1
= 1
๐น๐โ๐โ (
๐น๐+๐+๐โ1
๐น๐+๐+๐ โ๐น๐+๐โ1 ๐น๐+๐ )
๐โ๐
๐=1
. Case 2. ๐ is even
๐พ๐+๐(0, ๐ โ ๐) โ ๐พ๐(0, ๐ โ ๐) = 1
๐น๐โ๐โ ( ๐น๐โ1
๐น๐ โ๐น๐+๐+๐โ1 ๐น๐+๐+๐ )
๐โ๐
๐=1
โ 1
๐น๐โ๐โ ( ๐น๐โ1
๐น๐ โ๐น๐+๐โ1 ๐น๐+๐ )
๐โ๐
๐=1
= 1
๐น๐โ๐โ ( ๐น๐+๐โ1
๐น๐+๐ โ๐น๐+๐+๐โ1 ๐น๐+๐+๐ ).
๐โ๐
๐=1
โ
Page | 16
SUM WITH INDICES IN A.P.
Let โ be a natural number. We express certain sums in terms of the following two sums.
๐ฝ๐โ = โ 1 ๐นโ๐
๐
๐=1
, ๐พ๐โ = โ(โ1)๐ ๐นโ๐
๐
๐=1
NON-ALTERNATING SUM OF ORDER 1
We consider the problem of finding ๐ฝ๐โ(๐) in terms of ๐ฝ๐โ. ๐โ๐๐๐๐๐ 5: ๐ผ๐ ๐ > 0 ๐๐๐ โ|๐ ๐กโ๐๐ ๐ฝ๐โ(๐) = ๐ฝ๐+๐
โ โ โ ๐ฝ๐
โ โ .
๐๐๐๐๐:Let ๐ =๐
โ and ๐ = ๐โ then, ๐ฝ๐+๐
โ โ โ ๐ฝ๐
โ
โ= โ 1
๐นโ๐
๐+๐
๐=1
โ โ 1
๐นโ๐
๐
๐=1
= โ 1
๐นโ๐
๐+๐
๐=๐+1
= 1
๐นโ(๐+1)+ 1
๐นโ(๐+2)+ โฏ + 1
๐นโ(๐+๐โ1)+ 1 ๐นโ(๐+๐)
= 1
๐นโ+๐+ 1
๐น2โ+๐+ โฏ + 1
๐น(๐โ1)โ+๐+ 1 ๐น๐โ+๐
= ๐ฝ๐โ(๐).
โ
ALTERNATING SUM OF ORDER 1
We consider the problem of finding ๐พ๐โ(๐) in terms of ๐พ๐โ. ๐โ๐๐๐๐๐ 6: ๐ผ๐ ๐ > 0 ๐๐๐ โ|๐ ๐กโ๐๐
๐พ๐โ(๐) = { ๐พ๐
โ โโ ๐พ
๐+๐ โ
โ if ๐ is odd, ๐พ๐+๐
โ โ โ ๐พ๐
โ
โ if ๐ is even.
Proof:
Let ๐ =๐โ and ๐ = ๐โ then,
๐พ๐+๐โ โ ๐พ๐โ = โ(โ1)๐ ๐นโ๐
๐+๐
๐=1
โ โ(โ1)๐ ๐นโ๐
๐
๐=1
= โ (โ1)๐ ๐นโ๐
๐+๐
๐=๐+1
.
Once again we distinguish four cases.
Case 1. ๐ =๐
โ and ๐ are odd.
Page | 17 ๐พ๐โ(๐) = โ(โ1)๐
๐นโ๐+๐
๐
๐=1
= โ1
๐นโ+๐+ 1
๐น2โ+๐+ โฏ + 1
๐น(๐โ1)โ+๐+ โ1 ๐น๐โ+๐. ๐พ๐+๐โ โ ๐พ๐โ = โ (โ1)๐
๐นโ๐
๐+๐
๐=๐+1
= 1
๐นโ+โ๐+ โ1
๐น2โ+โ๐+ โฏ + โ1
๐น(๐โ1)โ+โ๐ + 1 ๐น๐โ+โ๐
= 1
๐นโ+๐+ โ1
๐น2โ+๐+ โฏ + โ1
๐น(๐โ1)โ+๐+ โ1 ๐น๐โ+๐
= โ ( โ1
๐นโ+๐+ 1
๐น2โ+๐+ โฏ + 1
๐น(๐โ1)โ+๐+ โ1 ๐น๐โ+๐)
= โ๐พ๐โ(๐).
Case 2. ๐ =๐
โ is odd, ๐ is even ๐พ๐โ(๐) = โ(โ1)๐
๐นโ๐+๐
๐
๐=1
= โ1
๐นโ+๐+ 1
๐น2โ+๐+ โฏ + โ1
๐น(๐โ1)โ+๐+ 1 ๐น๐โ+๐.
๐พ๐+๐โ โ ๐พ๐โ = โ (โ1)๐ ๐นโ๐
๐+๐
๐=๐+1
= 1
๐นโ+โ๐+ โ1
๐น2โ+โ๐+ โฏ + 1
๐น(๐โ1)โ+โ๐ + โ1 ๐น๐โ+โ๐
= 1
๐นโ+๐+ โ1
๐น2โ+๐+ โฏ + 1
๐น(๐โ1)โ+๐+ โ1 ๐น๐โ+๐
= โ ( โ1
๐นโ+๐+ 1
๐น2โ+๐+ โฏ + 1
๐น(๐โ1)โ+๐+ โ1 ๐น๐โ+๐)
= โ๐พ๐โ(๐) Case 3. ๐ =๐
โ is even , ๐ is odd ๐พ๐โ(๐) = โ(โ1)๐ ๐นโ๐+๐
๐
๐=1
= โ1
๐นโ+๐+ 1
๐น2โ+๐+ โฏ + 1
๐น(๐โ1)โ+๐+ โ1 ๐น๐โ+๐. ๐พ๐+๐โ โ ๐พ๐โ = โ (โ1)๐
๐นโ๐
๐+๐
๐=๐+1
= โ1
๐นโ+โ๐+ 1
๐น2โ+โ๐+ โฏ + 1
๐น(๐โ1)โ+โ๐+ โ1 ๐น๐โ+โ๐
= 1
๐นโ+๐+ โ1
๐น2โ+๐+ โฏ + 1
๐น(๐โ1)โ+๐+ โ1 ๐น๐โ+๐
= ๐พ๐โ(๐).
Page | 18 Case 4. ๐ = ๐
โ and ๐ are even ๐พ๐โ(๐) = โ(โ1)๐
๐นโ๐+๐
๐
๐=1
= โ1
๐นโ+๐+ 1
๐น2โ+๐+ โฏ + โ1
๐น(๐โ1)โ+๐+ 1 ๐น๐โ+๐. ๐พ๐+๐โ โ ๐พ๐โ = โ (โ1)๐
๐นโ๐
๐+๐
๐=๐+1
= โ1
๐นโ+โ๐+ 1
๐น2โ+โ๐+ โฏ + โ1
๐น(๐โ1)โ+โ๐+ 1 ๐น๐โ+โ๐
= 1
๐นโ+๐+ โ1
๐น2โ+๐+ โฏ + โ1
๐น(๐โ1)โ+๐+ 1 ๐น๐โ+๐
= ๐พ๐โ(๐).
โ
Page | 19
CHAPTER 3
SUM OF RECIPROCALS OF FIBONACCI NUMBERS WITH NEGATIVE INDICES
We use the conversion of negative Fibonacci numbers with negative indices to Fibonacci numbers with positive indices and then derive the identities for these numbers. We use the following notations.
โ๐ = โ 1 ๐นโ๐
๐
๐=1
, ๐๐= โ(โ1)๐ ๐นโ๐
๐
๐=1
We first write them in terms of ๐ฝ๐ and ๐พ๐. We use the formula ๐นโ๐= (โ1)๐+1๐น๐
stated in [3]. Then we have
โ๐ = โ 1 ๐นโ๐
๐
๐=1
= โ 1
(โ1)๐+1๐น๐ =
๐
๐=1
โ(โ1)๐+1 ๐น๐
๐
๐=1
= โ โ(โ1)๐ ๐น๐
๐
๐=1
= โ๐พ๐,
๐๐ = โ(โ1)๐ ๐นโ๐
๐
๐=1
= โ (โ1)๐ (โ1)๐+1๐น๐ =
๐
๐=1
โ(โ1)1 ๐น๐
๐
๐=1
= โ โ 1 ๐น๐
๐
๐=1
= โ๐ฝ๐.
โ
ORDER 1
We first consider the following sums:
โ๐(๐) = โ 1 ๐นโ๐โ๐
๐
๐=1
,
๐๐(๐) = โ(โ1)๐ ๐นโ๐โ๐
๐
๐=1
.
We write the above in terms of ๐ฝ๐(๐) and ๐พ๐(๐).
Page | 20
NON-ALTERNATING SUM
The non-alternating sum of Fibonacci numbers of negative indices of 1st order is,
โ๐(๐) = โ 1 ๐นโ๐โ๐
๐
๐=1
. We write this sum in terms of ๐พ๐(๐) .
โ๐(๐) = โ 1 ๐นโ๐โ๐
๐
๐=1
= โ 1
(โ1)๐+๐+1๐น๐ =
๐
๐=1
โ(โ1)๐+๐+1 ๐น๐
๐
๐=1
= (โ1)๐+1โ(โ1)๐ ๐น๐
๐
๐=1
= (โ1)๐+1๐พ๐(๐).
โ
ALTERNATING SUM
The alternating sum of Fibonacci numbers of negative indices of 1st order is, ๐๐(๐) = โ(โ1)๐
๐นโ๐โ๐
๐
๐=1
. We express this in terms of ๐ฝ๐(๐) .
๐๐(๐) = โ(โ1)๐ ๐นโ๐โ๐
๐
๐=1
= โ (โ1)๐ (โ1)๐+๐+1๐น๐ =
๐
๐=1
โ(โ1)๐+1 ๐น๐
๐
๐=1
= (โ1)๐+1โ 1 ๐น๐
๐
๐=1
= (โ1)๐+1๐ฝ๐.
โ
ORDER 2
We next consider the following 2nd order sums:
โ๐(0, ๐) = โ 1 ๐นโ๐๐นโ๐โ๐
๐
๐=1
,
๐๐(0, ๐) = โ (โ1)๐ ๐นโ๐๐นโ๐โ๐
๐
๐=1
,
โ๐(๐, ๐) = โ 1 ๐นโ๐โ๐๐นโ๐โ๐
๐
๐=1
,
๐ ๐(๐, ๐) = โ (โ1)๐ ๐นโ๐โ๐๐นโ๐โ๐
๐
๐=1
.
We express the above sums in terms of ๐ฝ๐(0, ๐), ๐พ๐(0, ๐), ๐ฝ๐(๐, ๐) and ๐พ๐(๐, ๐).
Page | 21
NON-ALTERNATING SUM
We consider the following 2nd order non-alternating sums.
โ๐(0, ๐) = โ 1 ๐นโ๐๐นโ๐โ๐
๐
๐=1
๐คโ๐๐๐ ๐ > 0,
โ๐(๐, ๐) = โ 1 ๐นโ๐โ๐๐นโ๐โ๐
๐
๐=1
๐คโ๐๐๐ 0 < ๐ < ๐.
We express the above sums in terms of ๐ฝ๐(0, ๐) and ๐ฝ๐(๐, ๐) respectively.
First we consider the sum,
โ๐(๐) = โ 1 ๐นโ๐๐นโ๐โ๐
๐
๐=1
= โ 1
(โ1)2๐+๐+2๐น๐ =
๐
๐=1
โ (โ1)๐ ๐น๐๐น๐+๐
๐
๐=1
= (โ1)๐โ 1 ๐น๐๐น๐+๐
๐
๐=1
= (โ1)๐๐ฝ๐(0, ๐).
โ We next consider,
โ๐(๐, ๐) = โ 1 ๐นโ๐โ๐๐นโ๐โ๐
๐
๐=1
= โ 1
(โ1)2๐+๐+๐+2๐น๐+๐๐น๐+๐
๐
๐=1
= โ (โ1)๐+๐ ๐น๐+๐๐น๐+๐
๐
๐=1
= (โ1)๐+๐โ 1 ๐น๐+๐๐น๐+๐
๐
๐=1
= (โ1)๐+๐๐ฝ๐(๐, ๐).
โ
ALTERNATING SUM
We consider the following 2nd order alternating sums.
๐๐(0, ๐) = โ (โ1)๐ ๐นโ๐๐นโ๐โ๐
๐
๐=1
๐คโ๐๐๐ ๐ > 0,
๐๐(๐, ๐) = โ (โ1)๐ ๐นโ๐โ๐๐นโ๐โ๐
๐
๐=1
๐คโ๐๐๐ 0 < ๐ < ๐.
Page | 22 We express the above sums in terms of ๐พ๐(0, ๐) and ๐พ๐(๐, ๐).
First we consider the sum, ๐๐(๐) = โ (โ1)๐
๐นโ๐๐นโ๐โ๐
๐
๐=1
= โ (โ1)๐ (โ1)2๐+๐+2๐น๐=
๐
๐=1
โ(โ1)๐+๐ ๐น๐๐น๐+๐
๐
๐=1
= (โ1)๐โ(โ1)๐ ๐น๐๐น๐+๐
๐
๐=1
= (โ1)๐๐พ๐(0, ๐).
โ Next we consider,
๐๐(๐, ๐) = โ (โ1)๐ ๐นโ๐โ๐๐นโ๐โ๐
๐
๐=1
= โ (โ1)๐
(โ1)2๐+๐+๐+2๐น๐+๐๐น๐+๐
๐
๐=1
= โ(โ1)๐+๐+๐ ๐น๐+๐๐น๐+๐
๐
๐=1
= (โ1)๐+๐โ (โ1)๐ ๐น๐+๐๐น๐+๐
๐
๐=1
= (โ1)๐+๐๐พ๐(๐, ๐).
โ
SUM WITH INDICES IN A.P.
Let โ be a natural number. We convert the following sums in terms of ๐ฝ๐โ and ๐พ๐โ. โ๐โ = โ 1
๐นโโ๐
๐
๐=1
, ๐๐โ = โ(โ1)๐ ๐นโโ๐
๐
๐=1
.
โ๐โ = โ 1 ๐นโโ๐
๐
๐=1
= โ 1
(โ1)โ๐+1๐นโ๐
๐
๐=1
= โ(โ1)โ๐+1 ๐นโ๐
๐
๐=1
= โ โ((โ1)โ)๐ ๐นโ๐
๐
๐=1
Page | 23
= {
โ โ(โ1)๐ ๐นโ๐
๐
๐=1
if โ is odd,
โ โ 1 ๐นโ๐
๐
๐=1
if โ is even.
= {โ๐พ๐โ if โ is odd,
โ๐ฝ๐โ if โ is even.
โ
๐๐โ = โ(โ1)๐ ๐นโโ๐
๐
๐=1
= โ (โ1)๐ (โ1)โ๐+1๐นโ๐
๐
๐=1
= โ(โ1)โ๐+1+๐ ๐นโ๐
๐
๐=1
= โ โ((โ1)โ+1)๐ ๐นโ๐
๐
๐=1
= {
โ โ 1 ๐นโ๐
๐
๐=1
if โ is odd,
โ โ(โ1)๐ ๐นโ๐
๐
๐=1
if โ is even.
= {โ๐ฝ๐โ if โ is odd,
โ๐พ๐โ if โ is even.
โ
NON-ALTERNATING SUM OF ORDER 1
We consider the problem of finding 1st order non-alternating sum of Fibonacci number with negative indices, โ๐โ(๐).
โ๐โ(๐) = โ 1 ๐นโโ๐โ๐
๐
๐=1
where โ > 0 and โ|๐.
We write the above sum in terms of ๐ฝ๐(๐) and ๐บ๐(๐).
Page | 24 โ๐โ(๐) = โ 1
๐นโโ๐โ๐
๐
๐=1
= โ 1
(โ1)โ๐+๐+1๐นโ๐+๐
๐
๐=1
= โ(โ1)โ๐+๐+1 ๐นโ๐+๐
๐
๐=1
= (โ1)๐+1โ(โ1)โ๐ ๐นโ๐+๐
๐
๐=1
= (โ1)๐+1โ((โ1)โ)๐ ๐นโ๐+๐
๐
๐=1
= (โ1)๐+1 {
โ(โ1)๐ ๐นโ๐+๐
๐
๐=1
if โ is odd,
โ 1
๐นโ๐+๐
๐
๐=1
if โ is even.
= {(โ1)๐+1๐พ๐โ(๐) if โ is odd, (โ1)๐+1๐ฝ๐โ(๐) if โ is even.
โ
ALTERNATING SUM OF ORDER 1
We consider the problem of finding 1st order non-alternating sum of Fibonacci number with negative indices, ๐๐โ(๐).
๐๐โ(๐) = โ (โ1)๐ ๐นโโ๐โ๐
๐
๐=1
where โ > 0 and โ|๐.
We express the above sum in terms of ๐ฝ๐(๐) and ๐บ๐(๐).
๐๐โ(๐) = โ (โ1)๐ ๐นโโ๐โ๐
๐
๐=1
= โ (โ1)๐
(โ1)โ๐+๐+1๐นโ๐+๐
๐
๐=1
= โ(โ1)โ๐+๐+1 ๐นโ๐+๐
๐
๐=1