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A FIBONACCI GENERALIZATION
BROTHER ALFRED BROUSSEAU
St. Mary's College, California
It is well known that the sum of any ten numbers of a Fibonacci sequence
is divisible by 11. For example, starting with 11, 15 and proceeding to 26,41,
67, 108, 175, 283, 458, 741, the sum of these ten terms is 1925 which is divisible by 11, the quotient being 175, the seventh member of the set of ten successive
terms.
This can be proved to hold in general, for if we start with any two
numbers a, b, the successive terms are a + b, a + 2b, 2a + 3b, 3a + 5b,
5a + 8b, 8a + 13b, 13a + 21b, 21a + 34b, the sum of which is 55a + 88b which
on being divided by 11 gives a quotient of 5a + 8b, the seventh term of the set
of ten terms.
This curious property might lead one to speculate on the possibility of
having various sets of successive terms of a generalized Fibonacci sequence
divisible by some common quantity.
To analyze the situation let us start with
terms Tt = a, T 2 = b, the usual Fibonacci relation.
T ,, = T + T A
n+i
n
n-i
Thus T3 = a + b,
T 4 = a + 2b,
T 5 = 2a + 3b,
T6 = 3a + 5b, T7 = 5a + 8b,
etc. It appears that the coefficients of a and b are Fibonacci numbers from
the sequence (1,1,2, 3,5, 8,13, • • •) with the general formula being
However, what we are considering is the sum of a certain number of terms of
this sequence. We want to find what:
n
]TTk
•= Tt + T2 + T3 + . , . + T n
k=i
equals in terms of a, b, and the Fibonacci numbers. Now it will be observed
that the coefficients of a in the summation are the Fibonacci numbers
171
172
A FIBONACCI GENERALIZATION
[April
1 9. 19 , 92 ,9 3 . - " ,' F n-3'
J the usual formula for the
o S F n-2 with an extra 1, so that by
sum of the Fibonacci numbers, the coefficient of a in the sum must be F ;
»
n'
the coefficient of b is simply the sum of the Fibonacci numbers up to and
including& F n-i*, so that this coefficient is Fn+i
, - 1 , Thus
n
E T,k = Fna + (Fn+i, - l)b'
v
,
k=i
We are looking for a common factor of this sum, no matter what values a and
b may have. Thus, we seek common factors of F and F , - 1. In the case
J
'
n
n+i
n = 10, F 10 = 55, F u - 1 = 88, so that the common factor is 11. A little
experimentation leads to the following table.
We begin with 10 and go to
higher values.
55
11 • 5
89-1
11-8
144
8 • 18
233-1
8 • 29
377
29 • 13
610-1
29 "• 21
987
21 • 47
1597-1
21 • 76
2584
76 • 34
4181-1
76 • 55
6765
55 • 123
10946-1
55 • 199
17711
199 • 89
28657-1
199 • 144
It appears that we have two cases. When n is of the form 4k + 2, the common factor is a Lucas number and the quotients a r e successive Fibonacci numb e r s ; while if n is of the form 4k, the common factor is a Fibonacci number
and the quotients successive Lucas numbers.
relations would seem to be as follows:
More precisely, the intuitive
1967]
A FIBONACCI GENERALIZATION
173
4k+2
T
£
j
=
F
4k+2a + (F4k+3 - l)b
=
L
2k+lF2k+la +
=
L
2k+l(F2k+la
=
L
2k+l T 2k+3
+
L
2k+i F 2k+2 D
F
2k+2 D )
and for the o t h e r c a s e :
4k
J2 T j
The f o r m u l a
F
= F
4k a
+
(F%+1 - Db
=
L
2kF2ka +
=
F
2k(L2ka + L2k+1b)
=
F
2 k ( F 2 k - l a + F2k*> +
=
F
2k( T 2k+l
+
F
2kL2k+lb
T
F
2k+la +
F
2k+2°)
2k+3)
F 2 n = F n L n i s well known. T h e r e a r e two other f o r m u l a s
4n+3 ~ 1 = L 2 n +iF 2 n + 2 and F 4 n + 1 - 1 = L 2 n + 1 F 2 n which need to be justified.
We f i r s t verify t h e m for s m a l l values of n.
F o r n = 0,
gives F 3 - 1 = 2 - 1
and hence i s 1 a s well.
o r 1 and L A F 2 = 1 « 1
n = 1,
the f i r s t f o r m u l a h a s on the left
right,
L 3 F 4 = 4 • 3 or
n,»
12.
the f i r s t
Thus the f i r s t f o r m u l a holds for s m a l l v a l u e s of
a s s u m e that the v a r i o u s f o r m u l a s hold up to F ^ + 2
a
Then
Adding
Then
Adding
For
F 7 - 1 = 13 - 1 o r 12 and on the
S i m i l a r l y , the second can be verified for t h e s e s m a l l v a l u e s .
in g e n e r a l .
formula
F
4n+i "
F
4n+i
1
=
L
2n+iFn
=
L
2n+i F 2n+l
F4n+3 - 1 = L2n+1F2n+2
F4n+4
= F2n+2L2n+2
F4n+5 - 1 = F2n+2L2n+3
^ d that F^Q = F ^ L
We now
holds
174
A FIBONACCI GENERALIZATION
April 1967
Hence, since t h e s e f o r m u l a s hold for s m a l l v a l u e s of n a s shown, it follows
that they can be proved to hold for any value of n by r e a s o n of m a t h e m a t i c a l
induction.
Thus the intuitive f o r m u l a s a r e s e e n to hold in g e n e r a l .
As a n u m e r i c a l i l l u s t r a t i o n of t h e s e f o r m u l a s , c o n s i d e r the s e r i e s s t a r t ing with a = 8, b = 1 1 .
k
T
k
*
T
1
8
8
2
11
19
Factorization
In Symbols
k
3
19
38
4
30
68
5
49
117
6
79
196
7
128
324
8
207
531
9
335
866
10
542
1408
11
12
877
2285
1419
3704
13
2296
6000
14
3715
9715
15
6011
15726
16
9726
25452
17
15737
41189
18
25463
66652
19
41200
107852
20
66663
174515
21
107863
282378
22
174526
456904
23
282389
739293
24
456915
1196208
25
739304
1935512
26
1196219
3131731
27
1935523
5067254
28
3131742
8198996
1 • 19
LjTg
1 • 68
F 2 (T 3 + T 5 )
49
• * • •
•
L3T5
3 • 177
F 4 (T 5 + T 7 )
11 - 128
L5T7
8 • 463
F 6 (T 7 + T 9 )
29 - 335
L7T9
2 1 - 1212
F 8 (T 9 +
76 • 877
L9Tlt
55 • 3173
F 1 0 ( T t l + T 13 )
199 • 2296
LUT13
144 • 8307
F 1 2 (T 1 3 + T 15 )
521 • 6011
L13T15
377 • 21748
F 14 (T 1B + T 17 )
Tn)