2 questions - series & inequalities

KLscilevothma

2 questions -- series & inequalities

1. By differentiate the function 1/(1-x), or otherwise, show that

inf
[sum] n2/2n = 6
n=1

2) Given Holder's Inequality http://mathworld.wolfram.com/HoeldersInequalities.html(equation 4)
show that (attached file)

Attachments

• 4 KB Views: 249
Last edited:

KLscilevothma

exam questions

The attatchment is ready now. These 2 questions appeared in my exam but I don't know how to do them.

metacristi

1.I do not know whether my solution is of help for you (it implies the basic knowledge of linear inhomogeneous recurrence) anyway I will present it.

Let

S=∑n=1 n2/2n (1)

and

S1(n)=∑k=1n k2/2k (2)

with S=limn->∞ S1(n) (3)

We have (equalities 3'):

n=1 ---> S1=1/(21)

n=2 ---> S1=6/(22)

n=3 ---> S1=21/(23)

n=4 ---> S1=58/(24)

n=5 ---> S1=141/(25)

n=6 ---> S1=318/(26)

Let now the general term of the sequence 1=a,6=a,21=a,58=a[ 4 ],141=a,318=a... be a[n].

The closed form for the sum S1(n) is:

S1(n)=a[n]/(2n) (4)

To find a[n] it must be observed that (equations 4'):

n=1 ---> a=1

n=2 ---> a=6

n=3 ---> a=21

n=4 ---> a[ 4 ]=58

n=5 ---> a=141

n=6 ---> a=318

and

a-a=1*2+3=5

a-a=5*2+5=15

a[ 4 ]-a=15*2+7=37

a-a[ 4 ]=37*2+9=83

a-a=83*2+11=177

a-a=177*2+13=367

a-a=367*2+15=749

..................................

a[n]-a[n-1]=2*{a[n-1]-a[n-2]}+(2n-1) (5)

If you have some experience with inhomogeneous sequences it's easy to find that we must seek for the closed form of a[n] a formula of the type:

a[n]=[A*n2+B*n+C]+D*2n

Introducing in (5) the equations 4' for n=1,2,3,4 and solving the system --->

A=-1
B=-4
C=-6
D=6

Therefore

a[n]={-1*n2-4*n-6}+6*2n (6)

Introducing (6) in (4) --->

S1(n)={{-1*n2-4*n-6}+6*2n}/(2n)} (4)

Finally

S1(n)=6-{(1*n2+4*n+6)/(2n)

and

S=6

Last edited:

metacristi

I don't understand what happened I used the edit feature...

Last edited:

metacristi

I don't understand what happened I used the edit feature...

Last edited:

metacristi

I don't understand what happened I used the edit feature...sorry.

Last edited:

Hurkyl

Staff Emeritus
Gold Member
1/(1 - x) = &Sigma;n=0..&infin; xn

Differentiate both sides of the equation with respect to x, see if that gives you any clues. What if the numberator was just n instead of n squared; could you use the derivative you just computed to evaluate the sum?

I haven't worked through the second one before... I'll see if I can give a hint once I have time to figure it out.

Edit: changed the lower case sigma to upper case sigma

Last edited:

KLscilevothma

metacristi, your method is pretty new to me but I'll try to understand that.
Originally posted by Hurkyl
1/(1 - x) = &sigma;n=0..&infin; xn
Differentiate both sides of the equation with respect to x, see if that gives you any clues. The question clears up a bit now. I'll try to work it out after my calculus exam.

For question 2, please take your time, there's no rush. I'll try to work out the answer 2 days later, after my exams. Again thanks.

KLscilevothma

I'm terribly sorry

that I forgot to mention that a1,a2...an>0 such that [sum]r=1..n ar = 1 for the second question.

metacristi

2.From what I've seen Holder's inequation hold only for (1/p)+(1/q)=1 and p,q >1 so that,I suppose,these are assumed true in the case of your problem too.

Let

Ar=(ar+1/ar)

Br=n-1/q

We have Ar,Br > 0 and Br <= 1 (if q > 1).

r=1 to n Ar*Br=∑r=1 to n n-1/q* (ar+1/ar) (1)

[∑r=1 to n (Ar)p]1/p=[∑r=1 to n (ar+1/ar)p]1/p (2)

[∑r=1 to n (n-1/q)q]1/q=(n*1/n)1/q=1 (3)

From Holder inequation,taking into account (1) (2) & (3) --->

[∑r=1 to n (ar+1/ar)p]1/p >= ∑r=1 to n Ar*Br=∑r=1 to n n-1/q* (ar+1/ar)

We have ar+1/ar >= 2--->

r=1 to n (ar+1/ar)p >= [∑r=1 to n n-1/q* (ar+1/ar)]p=n-p/q*[∑r=1 to n (ar+1/ar)]p

Last edited:

KLscilevothma

Thanks.
Originally posted by metacristi
Let

Ar=(ar+1/ar)

Br=n-1/q

We have Ar,Br > 0 and Br <= 1 (if q > 1).

&#8721r=1 to n Ar*Br=&#8721r=1 to n n-1/q* (ar+1/ar) (1)

[&#8721r=1 to n (Ar)p]1/p=[&#8721r=1 to n (ar+1/ar)p]1/p (2)

[&#8721r=1 to n (n-1/q)q]1/q=(n*1/n)1/q=1 (3)

From Holder inequation,taking into account (1) (2) & (3) --->

[&#8721r=1 to n (ar+1/ar)p]1/p >= &#8721r=1 to n Ar*Br=&#8721r=1 to n n-1/q* (ar+1/ar)

We have ar+1/ar >= 2--->

&#8721r=1 to n (ar+1/ar)p >= [&#8721r=1 to n n-1/q* (ar+1/ar)]p=n-p/q*[&#8721r=1 to n (ar+1/ar)]p
I don't understand the second last step. Why do we need ar+1/ar >= 2. (I know it can be proved by AM>=GM)

In fact I found out how to do it yesterday but didn't have time to post my proof. I used a similar approach and here's my proof.
http://www.angelfire.com/freak2/anywork/ine2.jpg

From what I've seen Holder's inequation hold only for (1/p)+(1/q)=1 and p,q >1 so that,I suppose,these are assumed true in the case of your problem too.
Actually it is part (b) of a long question. I think I better post the whole question here.
http://www.angelfire.com/freak2/anywork/inequality.jpg
I am going to do part (d) today, and will ask again if I can't do it.

bogdan

I think I got it...but it's quite simple...

if you know a)...then just put from 1/p+1/q=1, q=p/(p-1)...in d)...

sum[(Ar+1/Ar)^p]>n^(1-p)*[1+sum(1/Ar)]^p;
prove that sum(1/Ar)>n^2 this way...

sum(1/Ar)>n*(fact(1/Ar))^(1/n)=n*1/[fact(Ar)]^(1/n)>

> n/[sum(Ar)/n]=(n^2)/1...that's all...I think...

bogdan

I think I have a solution for a)...
a) is equivalent to

{{sum[(Ar+1/Ar)^p]}^(1/p)}*n^(1/q)>sum(Ar+1/Ar);

write n this way ...

n=sum(1^q)...

{{sum[(Ar+1/Ar)^p]}^(1/p)}*{[sum(1^q)]^(1/q)}>sum(Ar+1/Ar);

which is exactly holder ineq for (Ar+1/Ar) and (1)...(hope it's correct... metacristi

I don't understand the second last step. Why do we need ar+1/ar >= 2. (I know it can be proved by AM>=GM)
I intended to underline the fact that ar+1/ar > 0 but we don't need it,indeed,in the demonstration since ar are all positive.

metacristi

To prove a. suffices to write q as:

q=p/(p-1) and by studying the function q=f(p) --->

-when p < 0 ---> q is always greater than 0 ---> pq < 0.

-when p belongs to (0,1) ---> q is always negative ---> pq < 0.

-when p belongs to (1,+∞) that's it when p > 1 we have q greater than 0 ---> pq > 0 q.e.d.

My solutions to c. and d. (slightly different than those of Bogdan) are:

c.

You must observe that:

[a1+..+an]*[1/a1+...+1/an] ≥ n+n[C]2*2=n+{n!/[(n-2)!*2!)]}*2=n^2

where I used:

{ar/ak}+[1/{ar/ak}] ≥ 2 (obviously (A+1/A) ≥ 2 when A > 0))

and n[C]2=combinations of n,2=n!/[(n-2)!*2!)]

d.

From:

[sum]r=1 to n ar = 1,

b. by replacing q with p/(p-1),

c. [sum] 1/ar ≥ n2 ---> [sum] ar+1/ar=[sum] ar+[sum] 1/ar ≥ 1+n2,

---> the required inequality.

[edited to change the layout of the demonstration]

Last edited:

KLscilevothma

Originally posted by Hurkyl
1/(1 - x) = &Sigma;n=0..&infin; xn

Differentiate both sides of the equation with respect to x
1/(1-x)2 = &Sigma;n=0..&infin;nxn-1
sub x=1/2, then multiply both sides by 1/2
&Sigma;n=0..&infin;n/n2

Take the 2nd derivatives on both sides of
1/(1 - x) = &Sigma;n=0..&infin; xn
2/(1-x)3=&Sigma;n=0..&infin;(n2-n)xn-2
sub x = 1/2 and multiply both sides by 1/4
The result follows.

edit: metacristi, I don't know inhomogeneous sequences

Last edited:

KLscilevothma

Thanks

Originally posted by bogdan
I think I got it...but it's quite simple...

if you know a)...then just put from 1/p+1/q=1, q=p/(p-1)...in d)...

sum[(Ar+1/Ar)^p]>n^(1-p)*[1+sum(1/Ar)]^p;
prove that sum(1/Ar)>n^2 this way...

sum(1/Ar)>n*(fact(1/Ar))^(1/n)=n*1/[fact(Ar)]^(1/n)>

> n/[sum(Ar)/n]=(n^2)/1...
Bogdan, I used the same method as you do (see my post above yours).

I now understand these 2 questions, thanks for your help.

Yes, it's simple. I knew how to do part a, b and c when I was at home and used about 15 minutes to finish them (exclude part d), but not in exam.. I was too nervous, I only had 20 minutes to think whether to choose this question or the other and finish it. I only had 1 day to revise for this algebra exam while I had put aside algebra for 6 months and studied calculus. (finding excuses to cover up my stupidity! )

Physics Forums Values

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving