0

Were they stupid?
 in  r/mathmemes •  17h ago

Wouldn't the new design just get shot at differently?

2

Is the Google Pixel 11 Pro XL worth buying?
 in  r/pixel_phones •  17h ago

Get the regular pro unless you have big hands.

1

Literally how ?🥲
 in  r/the_calculusguy •  19h ago

Ah, yes, how physics departments do math.

1

Does anyone know a simple proof to show that a_1 + 2a_2 + 4a_4 +8a_8 + ... is bounded when the a_n's are all positive and decreasing?
 in  r/askmath •  1d ago

No I need to prove the cauchy condensation theorem/cuachy criterion/2n test as well.

1

Who starts a textbook this way?
 in  r/physicsmemes •  1d ago

All that just for Jaynes to fix it.

1

Does anyone know a simple proof to show that a_1 + 2a_2 + 4a_4 +8a_8 + ... is bounded when the a_n's are all positive and decreasing?
 in  r/askmath •  2d ago

I don't care about equivalence in difficulty, I am trying to find the method for finding the slowest growing positive decreasing sequence of partial sums which is unbounded. Aka the slowest divergent non-negative series. That value seems to be S 1/n. S 1/np where p>1 converges.

The reason 1/n does not converge is because it goes to 0 slower than 2p increases. This is what I wanted to find out, and from what I've seen the only way to find that out is to use 2p (2a_np) without using improper integrals. Improper integrals also seem to be less general.

When a_n is 1/np , then 2p (2a_np) = [2(1-p)]n. Since [21-p]n is a geometric series, that converges when 21-p < 1 and diverges when 21-p >\ 1. Thus 1/np converges when 1-p < 0 <=> p > 1.

This is helpful because the infinite limit of 1/n = 0, so we need another way to determine whether or not partial sums of it converge. The 2n test produces a closed form geometric sequence of partial sums and we know both that it is an upper bound for the partial sums of 1/n and how to test it for convergence. Its convergence then implies the convergence 1/np , and its divergence then implies the divergence of 1/np -- even without knowing the closed form limit of the partial sums of 1/n.

I found a better explamation in a different book:

1

New fear unlocked
 in  r/Funnymemes •  2d ago

Sunlight too...

-1

Unfortunate experience many minority children experience. POC boy is completely ignored by a women entertaining other kids.
 in  r/TikTokCringe •  2d ago

I noticed that Protestant democrats are the worst racists. Manifest Destiny types.

3

ಥ⁠‿⁠ಥ
 in  r/sadposting •  2d ago

A thick hoodie and rain is not bad.

2

666
 in  r/DarkPsychology666 •  2d ago

And it's not even true. People who like someone more than anyone else might get nervous and be too scared to do anything.

1

I thought I was getting myself an upgrade, well it isn't.
 in  r/pixel_phones •  2d ago

That dissapoints me. It clearly had a downgraded display. I expect all pixel A series devices to be as good as the 4a 5g.

1

He was moved to tears by the wedding surprise, an RTX 5090 graphic card
 in  r/interesting •  2d ago

stock market... but a gift is still nice.

0

As a woman (and probably on behalf of the gays), yes, please.
 in  r/SipsTea •  2d ago

I'll say one thing: errections in pants are painful. I have wondered for a while why pants and shorts don't have protuding "penis pouches" that extend out of the pants to allow errections without pain.

1

Does anyone know a simple proof to show that a_1 + 2a_2 + 4a_4 +8a_8 + ... is bounded when the a_n's are all positive and decreasing?
 in  r/askmath •  2d ago

On the next page they say that from this lemma, if S_N is bounded, then S_2K is bounded and therefore T_K is bounded. Conversly if T_K is bounded, then S_2K+1-1 is bounded and (by induction) 2K+1-1 >\ K + 1, so S_N is bounded.

1

Does anyone know a simple proof to show that a_1 + 2a_2 + 4a_4 +8a_8 + ... is bounded when the a_n's are all positive and decreasing?
 in  r/askmath •  2d ago

No, the paper says the criterion is used to test whether a positive and decreasing series converges... the proof says the task is to show thst S_N is bounded iff T_k is bounded. So I asked how to show T_k is bounded, which is the task. The series S (a_n), with partial sums S_N, converges only if S (2k(a_2k), with partial sums T_K, converges. The proof is to show S (2k(a_2k)) converges, and thus to prove that T_K is bounded.

To do this, they use a lemma: for any natural number K, S_2K+1-1 <\ T_K <\ 2S_2k . They prove this lemma by using induction on K, which is what I'm struggling to follow. The initial case of K=0 is simple: S_1 <\ T_0 <\ 2S_1 <=> a_1 <\ a_1 <\ 2a_1 , since a_1 is non-negative.

Assuming K and showing it holds for K+1 is where I get lost.

Perhaps I should show you the next page:

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Does anyone know a simple proof to show that a_1 + 2a_2 + 4a_4 +8a_8 + ... is bounded when the a_n's are all positive and decreasing?
 in  r/askmath •  3d ago

Needs to be an infinite sequence. Here is a photo of a proof, I simply can't follow it.

1

Does anyone know a simple proof to show that a_1 + 2a_2 + 4a_4 +8a_8 + ... is bounded when the a_n's are all positive and decreasing?
 in  r/askmath •  3d ago

Yeah, that's like the indefinite integral test. However, I'd like to use the cauchy criterion on the harmonic series.

8

Thy say the the king
 in  r/the_calculusguy •  3d ago

I love this.

2

Does anyone know a simple proof to show that a_1 + 2a_2 + 4a_4 +8a_8 + ... is bounded when the a_n's are all positive and decreasing?
 in  r/askmath •  3d ago

Yeah, all I said about (a_n) is that it is all positive and decreasing. I'm not even talking about the sum of the terms of (a_n), I'm just talking about a sequence. The sum of the terms of (a_n) only converges if the sum of 2n a_2n converges

However, to be justified in saying that I must first prove that to be true by finding out when 2n a_2n is bounded and when it isn't. It is this part that I am struggling with, because I am finding it not so easy to compare the rates of growth between 2n and different possibls (a_n)'s.

4

Does anyone know a simple proof to show that a_1 + 2a_2 + 4a_4 +8a_8 + ... is bounded when the a_n's are all positive and decreasing?
 in  r/askmath •  3d ago

Well yeah, but I wanted to go the other direction where I use the Cauchy criterion to find out that any p>1 causes convergence. Others have said a ratio test. I haven't tried that yet.

1

Does anyone know a simple proof to show that a_1 + 2a_2 + 4a_4 +8a_8 + ... is bounded when the a_n's are all positive and decreasing?
 in  r/askmath •  3d ago

I am given no task. I am trying to prove and understand why that exact result is true, in order to see why 1/n is the cut off for divergence.

Right now I only know that it happens, but I don't know why.

r/askmath • • 3d ago

Analysis Does anyone know a simple proof to show that a_1 + 2a_2 + 4a_4 +8a_8 + ... is bounded when the a_n's are all positive and decreasing?

8 Upvotes

I'm not sure how to find or compare the rate of growth of 2n with any positive decreasing sequence, in order to find the exact cutoff point where some positive decreasing sequence decreases slower than 2n increases.

How do I find that exact cutoff point in this situation?

The series in the title only converges when the all positive decreasing sequence (a_n) decreases faster than 2n increases.