[PDF] Simplifying finite sums - CMU



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Simplifying finite sums - CMU

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Simplifying nite sums

Misha Lavrov

ARML Practice 1/26/2014

Warm-up

1. Find 1 + 2 + 3 + + 100. (The story goes that Gauss was given this problem by his teacher in elementary school to keep him busy so he'd quit asking hard questions. But he gured it out in his head in about ten seconds.) 2. (2003 AIME I.) One hundred c oncentriccircles with radii

1;2;3;:::;100 are drawn in a plane. The smallest circle is

colored red, the strip around it green, and from there the colors alternate. What fraction of the total area of the largest circle is colored green?

Warm-up

Solutions

1.

The classic a rgumentgo eslik ethis:

1 + 2 + 3 ++ 100

+ 100 + 99 + 98 ++ 1101 + 101 + 101 ++ 101

IfSis the sum, 2S= 100101 = 10100, soS= 5050.

2. The a reaof a strip b etweenthe circle of radius rand the circle of radiusr+1 is(r+1)2r2= (2r+1), which we can rewrite as (r+ 1)+r. Then x=1002992+ 982972++ 221210000

100+ 99+ 98+ 97++ 2+10000

505010000= 0:505:

Basic summations

1.

Arithmetic series:

n X k=1k= 1 + 2 ++n=n(n+ 1)2 =n+ 1 2 In general, given an arithmetic progression that starts ata, ends atz, and hasnterms, its sum isna+z2 2.

Geometric se ries:fo rr6= 1,

n1X k=0r k= 1 +r+r2++rn1=rn1r1:

As a special case,

Pn1 k=02k= 2n1.

Exchanging double sums

Consider the sumS=Pn1

k=0k2k. We will evaluate this sum as follows:n1X k=0k2k=n1X k=0k1X `=02 k=n1X `=0n1X k=`+12 k:Having reordered the two sums, we rst evaluate the inner one: n1X k=`+12 k=n1X k=02 k`X k=02 k= (2n1)(2`+11) = 2n2`+1:Now the outer sum is also easy: n1X `=0(2 n2`+1) =n2n2n1X `=02 `= (n2)2n+ 2:

Exchanging double sums

Consider the sumS=Pn1

k=0k2k. We will evaluate this sum as follows:n1X k=0k2k=n1X k=0k1X `=02 k=n1X `=0n1X k=`+12 k:Having reordered the two sums, we rst evaluate the inner one: n1X k=`+12 k=n1X k=02 k`X k=02 k= (2n1)(2`+11) = 2n2`+1:Now the outer sum is also easy: n1X `=0(2 n2`+1) =n2n2n1X `=02 `= (n2)2n+ 2:

Exchanging double sums

Consider the sumS=Pn1

k=0k2k. We will evaluate this sum as follows:n1X k=0k2k=n1X k=0k1X `=02 k=n1X `=0n1X k=`+12 k:Having reordered the two sums, we rst evaluate the inner one: n1X k=`+12 k=n1X k=02 k`X k=02 k= (2n1)(2`+11) = 2n2`+1:Now the outer sum is also easy: n1X `=0(2 n2`+1) =n2n2n1X `=02 `= (n2)2n+ 2:

Practice with exchanging double sums

1.

W edene the n-th harmonic numberHnby

H n=nX k=11k =11 +12 ++1n

Express the sum

Pn k=1Hkin terms ofHn. 2.

Things will get trickier when y oudo t histo

Pn k=1k2. (Recall that Pn k=1k=n(n+1)2

Exchanging double sums

Solutions

1. nX k=1H k= (n+ 1)Hnn: 2.

Let n=Pn

k=0k2. When we expand this out into two sums, switch the sums, and simplify, we get back n=nX `=1 n+ 1 2 2 =2n3+ 3n2+n4 12 n X `=1` 2: We don't yet know how to simplify the last sum, but since it is just 12 n, we can solve the equation fornto get n=n(n+ 1)(2n+ 1)6

Review of binomial coecients

Recall that

n r=n!r!(nr)!=n(n1)()(nr+1)r!. These show up in

Pascal's triangle:K0

0O1K1 0O1K1 1O1K2 0O1K2 1O2K2 2O1K3 0O1K3 1O3K3 2O3K3 3O1K4 0O1K4 1O4K4 2O6K4 3O4K4

4O1The key point today is the identity

n r=n1 r+n1 r1, orn r=n+1 r+1n r+1. This lets us make many sums telescope: e.g., n1X k=0k=n1X k=0 k 1 =n1X k=0 k+ 1 2 k 2 =n 2

Sums of binomial coecients

In general, we have:

n1X k=0 k r =n1X k=0 k+ 1 r+ 1 k r+ 1 =n r+ 1 0 r+ 1 We can use this to evaluate sums of arbitrary polynomials. 1.

W ritey ourdegree- dpolynomial inkas a sum ofk

0;k

1;:::;k

d. For example,k2= 2k 2+k 1: 2.

No ww ecan evaluate the sum:

n X k=1k

2= 2nX

k=1 k 2 +nX k=1 k 1 = 2n+ 1 3 +n+ 1 2 3.

Then simplify: 2

n+1 3+n+1 2=16 (2n3+ 3n2+n).

Practice with binomial coecients

1.

Use this technique to nd a fo rmulafo r

Pn k=1k3. 2. Here is a systematic metho dof writing a p olynomialP(x) as a 0x 0+a1x

1++adx

d: 2.1

I fthe p olynomialis P(x) and has degreed, write

P(0);P(1);:::;P(d) in a row.

2.2 On the next line, write do wnthe dierences: b etweenand below two numbersaandbwriteba. 2.3 Rep eatstep 2 to the ro wof dierences, and k eepgoing until a row with only one number is left. 2.4

T hec oecientsof

x

0;:::;x

dare the rst entries in each row.

Use this method to nd

Pn k=1k4. 3.

Why do esthe metho dab ovew ork?

Practice with binomial coecients

Solutions

1.

W ritingk3as 6k

3+ 6k 2+k

1, we get

n X k=1k

3= 6n+ 1

4 + 6n+ 1 3 +n+ 1 2 =n2(n+ 1)24 2.

The ro wsof dierences w eget a re:

0 1 16 81 256

1 15 65 175

14 50 110

36 60
24

Sok4= 24k

4+ 36k

3+ 14k

2+k

1, and

n X k=1k

4= 24n+ 1

5 + 36n+ 1 4 + 14n+ 1 3 +n+ 1 2

The dierence operator

Given any functionf, fis another function dened by f(x) =f(x+ 1)f(x). (A thing that turns functions into other functions is called an operator. is called thedierence operator.)

The basic identity we rely on is this:

b1X k=af(k) =f(b)f(a): To simplify any nite sum whatsoever, all we need to do is nd a functionfsuch that fis the function we're summing. This is what we did in the previous section: we discovered that xx r=x r1, which let us solve any sum with binomial coecients in it.

Dierence operator problems

1.

Compute (2

x), (x2x), and (x22x). 2. Use #1, and the basic rule th at( f+g) = f+g, to nd n1X k=02 k;n1X k=0k2k;andn1X k=0k 22k:
3. Let Fndenote then-th Fibonacci number, dened byF0= 0, F

1= 1, andFn=Fn1+Fn2forn2. Find a formula for

n1X k=0F k: 4.

Prove that fo rany function f,

n1X k=0kf(k) =nf(n)nX k=1f(k):

Dierence operator problems

Solutions

1. (2 x) = 2x, (x2x) = (x+ 2)2x, and (x22x) = (x2+ 4x+ 2)2x. 2.

F rom#1, w ecan deduce that (( x24x+ 6)2x) =x22x.

Thereforen1X

k=0k

22k= (k24k+ 6)2k6:

The other two sums are similar, but easier.

3.

W ehave Fn=Fn+1Fn=Fn1, soFn= Fn+1, and

thereforen1X k=0F n=Fn+1F1=Fn+11: 4.

Sum b othsides of ( kf(k)) =f(k+ 1)kf(k).

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