an exploration of difference triangles
DESCRIPTION
An Exploration Of Difference Triangles. By Adrian Ferenc April 3, 2008 Occidental College. Outline. I. Introduction II. Difference Triangles III. Increasing Triangles IV. Row Sum Triangles V. Open Questions. Axiomatically Defined Structures. Algebraic Group Equivalence Relation - PowerPoint PPT PresentationTRANSCRIPT
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An Exploration Of Difference Triangles
By Adrian Ferenc
April 3, 2008
Occidental College
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Outline
I. Introduction II. Difference Triangles III. Increasing Triangles IV. Row Sum Triangles V. Open Questions
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Axiomatically Defined Structures
Algebraic Group
Equivalence Relation
Topology
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Pascal’s Triangle
11
11
11
11
11
1
cba
c
ba
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Pascal’s Triangle Continued
15101051
14641
1331
121
11
1
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Difference Triangles
A triangle is a Difference Triangle if it adheres to the following condition:
For any .
All entries are nonnegative integer values.
cbac
ba,
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An Example
Sloane’s A059346
36 51 72 102 145 207 297 429
15 21 30 43 62 90 132
6 9 13 19 28 42
3 4 6 9 14
1 2 3 5
1 1 2
0 1
1
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A Blank Triangle
1,1
2,22,1
3,33,23,1
4,44,34,24,1
c
cc
ccc
cccc
jijiji ccc ,1,11,
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Removing any number of the end columns still results in a difference triangle.
Multiplying our triangle by a “scalar” results in a difference triangle.
If in a triangle there exists at least one element in every row that is divisible by some k, then every element of the triangle is divisible by k.
Some Facts About Difference Triangles
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Increasing TrianglesIf in a difference Triangle, for all
Or, equivalently, in any row m,
Then this triangle is an Increasing triangle.
cabc
ba,
mmmm ccc ,,2,1
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Increasing Triangles Continued
In the same way Pascal’s Triangle was constructed by only using we can similarly construct an increasing triangle
...1,1,1,1,1
1
1
1
1
1
21
421
8421
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Facts About Increasing Triangles We can add together any two increasing triangles. Since in any row m, , if we sum up
all the elements in a row we get:
which equals .
mmmm ccc ,,2,1
m
kmmmmmmmmmk ccccccc
11,1,11,21,31,11,2,
1,11,1 mmm cc
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An Example
What if we choose the leftmost column to be the numbers 1, 1, 2, 3, 5, 8, …, i.e., the Fibonacci Sequence.
Extrapolating on this we can see that:
1
21
532
13853
34211385
112
12
nn
n
nkn fff
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Row Sum Triangles
A Triangle is a Row Sum triangle if it is a difference
triangle and, for some infinite sequence ,
row n in the Triangle sums to, so for any row m,
,,, 321 aaa
na
m
immi ac
1,
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So Why the Restriction?
Let’s remove the absolute value restriction and see what happens…
For any .cabc
ba,
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cabc
ba,
Let’s have our rows sum up to the numbers 1, 1, 2, 3, 5, etc…, i.e., the Fibonacci numbers:
Row 1Rows 1 & 2
Rows 1 – 3
So there is no integral solution.
1
1
10
1
10
yxx
34
43
1
22
y
y
xy
yx
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Example 1
Let’s have our row sum sequence be 1,1,1,…
Increasing Unique Multiply by k 1
10
100
1000
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“Example 2”
What if we try the sequence 1,2,3,4,…
For row 1 we get 1.
For row 2 we get x, y where x + y = 2, x - y = 1.
Clearly Nyx,
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Nonexistent Row Sum Triangles
So for such a sequence to correspond to a row sum triangle.
Also Similarly, our sequence cannot be infinitely decreasing.
)2(mod21 aa
21 aa
)2(modyxyx
yxyx
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Our row sum sequence can however “jiggle,” or increase and decrease.
1111
01010
0110
010
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Example 2
Let’s try the row sum sequence 1, 3, 5, 7,…
Not an increasing triangle. Unique?
1
12
320
5200
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Non-Uniqueness
1
12
320
3022
30024
300026
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Example 2′ Let’s try the row sum sequence 0, 2, 4, 6,…
0
11
121
3201
52001
1
12
320
5200
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Back To Pascal’s Triangle
15101051
14641
1331
121
11
1
Note that here the rows sum up to 1, 2, 4, 8, … or12 k
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Example 3
Let’s have our sequence be 2, 4, 8, 16, …
2
31
521
9421
n
k
nk
1
1 122
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Example 4
Let’s have our sequence be 1, 3, 9, 27, …
1
21
423
8469
n
k
nkkn
1
1)1( 323
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Example 5, A Generalization
So we’ve seen and .
Are we able to generalize this to ?
n2 13 n
nN
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Yes!
In fact we can do more!
We can create a row sum triangle whose rows sum to:
nn NN N
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nn NN
1
1 1
1, )1(1
1
ij
k i
kjkji iN
k
ijc
We can create a row sum triangle for the above sequence by:
N
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“Proof”
Outline of proof: Part 1: Show
Part 2: Show that this Triangle is increasing by showing that
This tells us that
0, yxc
1,1,1, yxyxyx ccc
1,11,11
,
nnn
n
knk ccc
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Part 3: Here we show that:
Combining parts 1 – 3 we get:
“Proof” Continued
11,1
i
mm iNc
11,1 )1(
i
mmm iNc
n
k i i
nnnnnk NNiNiNc
1 1 1, )()()1(
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Taste Of Proof
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Example 6 Let’s try a different type of sequence. The Fibonacci Sequence is defined recursively as follows:
.1,01 2101221 ffffffff kkk
)3(mod1
)3(mod0
)3(mod2
3
2
1
,
ijiff
ijiff
ijiff
c
i
i
i
ji
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The Fibonacci Triangle
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Fibonacci Triangle Continued Is it unique? No
Since 12 kkk fff
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Back To Pascal’s Triangle
Zalman Usiskin, “Square Patterns in the Pascal Triangle”
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Back To Fibonacci Triangle Does the same identity hold in the Fibonacci Triangle?
No… But
)5)(21)(5()8)(8)(8(?
13)5)(21)(5()8)(8)(8(
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Fibonacci Identity
Will this always hold?
|ace-bdf| =g iff
de
cgf
ba
1,1,1
,1,,1
1,11,
jiji
jijiji
jiji
cc
ccc
cc
)3(mod0 ij
iiiiii ffffff 23
322
21
3
iiiiii ffffff 23
322
21
3
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Another Fibonacci Identity
121)()( 122
122
1 nnn
nnnn ffffff
fgh
ejia
dcb
121 ijacegbdfh n
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Open Questions
What other conditions prohibit a sequence from corresponding to a row sum triangle and can all sequences that do not correspond to a row sum triangle be categorized?
What other ways can Fibonacci identities be obtained from the Fibonacci Triangle?
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Open Questions
What is the ratio of the number of difference triangles created by using the numbers {0, 1, 2, … n-1, n} to the number of difference triangles created by using only the numbers {0, 1, 2, … n-1}?Note that a triangle randomly generated by just 0s
and 1s has 0 probability of being a difference triangle.
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Thanks
I’d like to thank Professors Lengyel, Sundberg and Tollisen, as well as the rest of Occidental’s Mathematics Department.
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Works Cited
Chen Chuan-Chong and Koh Khee-Meng, Principles and techniques on Combinatorics. World Scientific Publishing Company, September 1992.
V.E. Hoggatt and W. Hansell, The hidden hexagon squares, Fibonacci Quarterly, 9 (1971) 120.
A. F. Horadam, A Generalized Fibonacci Sequence, The American Mathematical Monthly, Vol. 68, No. 5. (May, 1961), pp. 455-459.
Kenneth Rosen, Discrete Mathematics and its Applications. McGraw-Hill; 5 Edition, April 22, 2003.
N.J.A. Sloane, Encyclopedia of Integer Sequences <http://www.research.att.com/~njas/sequences/>
Zalman Usiskin, Square Patterns in the Pascal Triangle, Mathematics Magazine, Vol. 46, No. 4. (Sep., 1973), pp. 203-208.
Eric W. Weisstein. MathWorld – A Wolfram Web Resource. <http://mathworld.wolfram.coml>