Careful thinking to update our minds

Calculus Level 3

Consider the following statements:

Statement p: 1 1 d x x = 0 ~~~~\displaystyle \int_{-1}^{1} \dfrac{d x}{x} = 0

Statement q: n lim ~~~~\Large ^{\lim}_{n \to \infty} n n d x x = 0 \displaystyle \int_{-n}^{n} \dfrac{d x}{x} = 0

Are they respectively true or false?

True and True True and False All descriptions are invalid False and False False and True

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1 solution

Lu Chee Ket
Dec 21, 2015

Let's start with d x x = ln x + C \displaystyle \int \dfrac{d x}{x} = \ln \color{#3D99F6}{| x |} + C while integrate across 0!

2 3 d x x = ln 3 ln 2 = ln 3 2 \int_{-2}^{3} \dfrac{d x}{x} = \ln 3 - \ln 2 = \ln \frac32 = 2 3 d x x \int_{2}^{3} \dfrac{d x}{x}

2 3 d x x = ln 3 ln 2 = ln 3 2 \int_{2}^{-3} \dfrac{d x}{x} = \ln 3 - \ln 2 = \ln \frac32 = 2 3 d x x \int_{-2}^{-3} \dfrac{d x}{x}

Both are correct! The second one can be realized from 3 2 d x x = ( ln 2 ln 3 ) = ln 3 2 . - \int_{-3}^{2} \dfrac{d x}{x} = -(\ln 2 - \ln 3) = \ln \frac32. As thought as skipping area of n n d x x \int_{-n}^{n} \dfrac{d x}{x} of zero! They are reconnected after skipped. T h i s t e l l s t h a t 1 1 d x x = 0. \color{#3D99F6}{This~tells~that~ \int_{-1}^1 \dfrac{d x}{x}= 0.}

1 1 d x x \int_{-1}^1 \dfrac{d x}{x} = \Large^\lim_{\epsilon \to 0}[\int_{-1}^{\epsilon^-} \dfrac{d x}{x} + \int_{\epsilon^+}^1 \dfrac{d x}{x}] = \Large^\lim_{\epsilon \to 0} (\ln \epsilon - \ln \epsilon) = 0.

Note that 1 0 d x x + 0 1 d x x \int_{-1}^{0^-} \dfrac{d x}{x} + \int_{0^-}^{1} \dfrac{d x}{x} = indeterminate but not preserving the integral of 1 1 d x x . \int_{-1}^1 \dfrac{d x}{x}.

d x x = ln x + C \int \dfrac{d x}{x} = \ln x + C when x 0 x \geq 0 while d ( x ) x = ln ( x ) + C \int \dfrac{d (-x)}{-x} = \ln (-x) + C when x < 0. x < 0.

To conclude with simplicity, we just need to write d x x = ln x + C , \int \dfrac{d x}{x} = \ln | x | + C, where integrate crossing zero with zero on the line is inclusive! Hence, statement q is proven to be True! \Large^\lim_{n \to \infty} \int_{-n}^n \dfrac{d x}{x} = n lim [ n ϵ d x x + ϵ + n d x x ] \Large^{\lim}_{n \to \infty}[\int_{-n}^{\epsilon^-} \dfrac{d x}{x} + \int_{\epsilon^+}^{n} \dfrac{d x}{x}] = n lim ( ln ϵ n + ln n ϵ + ) \Large^{\lim}_{n \to \infty} (\ln \frac{\epsilon^-}{-n} + \ln \frac{n}{\epsilon^+}) = ln 1 = 0. \ln 1 = 0.

ϵ 0 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^{\Large \epsilon \to 0} ϵ 0 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^{\Large \epsilon \to 0}

Hence, statement p is also proven to be True!

Statement p is conclusive which includes statement q rather than just for both infinities. To think about ln ( 1 ) , \ln (-1), I think at least the difference from contour integral is there is a radius of another ϵ \epsilon from singularity zero for the complex variable analysis. In the context of real integral like n n d x x , \int_{-n}^n \dfrac{d x}{x}, there is at least an on the line by the singularity zero! ln ( 1 ) \ln (-1) is defined by me as 1 β π \sqrt{-1} \beta \pi where β \beta = {..., -3, -1, 1, 3, 5, 7, ...}; but since an on the line is not synonym to passing by, an outcome of ln ( 1 ) \ln (-1) for n n d x x \int_{-n}^{n} \dfrac{d x}{x} is n o t not necessarily true. The trick is d ( x ) x = d x x \int \dfrac{d (-x)}{-x} = \int \dfrac{d x}{x} ought to be smart to make automatic selection with regard to singularity zero!

One interesting is ln x \ln x = 1 x ϵ ϵ \displaystyle \frac{1 - x^{-\epsilon}}{\epsilon} where ϵ 0. \epsilon \to 0. You could substitute values to see.

In one round, we actually have angles of -180 ^\circ < θ \theta \leq 180 ^\circ to have zero fault for a complex number to the power of another complex number, regardless of fault according to general number. Provided angles are highly restricted to the range mentioned above, having 2 π 2 \pi as angle which duplicated the angle 0 and etc. could be a fault of mistake. Therefore, I am doubted by Cauchy actually.

From point of view of seeing 1 1 d x x \int_{-1}^1 \dfrac{d x}{x} = indeterminate x o r xor 1 1 d x x \int_{-1}^1 \dfrac{d x}{x} = 0, with x = 0 occupying no width for area, but a Dirac's infinity, I would think that Dirac obtained his intuition from here but such infinity in the context of our discussion here does not have gender of positive or negative actually. For our case, we can still imagine that i t it neutralized itself into zero as well! Therefore, 0 would be reasonable because we should not suddenly let it to produce something which cannot be certain or cannot be determined but anything like ln 2 \ln 2 or etc. My instinct up to here tells me that some other things must only arise from our improper treatments.

Fair and lovely!

Answer: F a l s e a n d F a l s e \boxed{False~and~False} however as d x d x cannot be 0 and therefore value of a d A d A or 1 0 d x \frac10 d x does not exist.

Generalizing a subject (allows banned expressions):

n × 0 = 0 n = 0 0 n \times 0 = 0 \implies n = \frac00 = indeterminate

1 0 × 0 = 1 \frac10 \times 0 = 1 while 1 0 × 0 × 0 \frac10 \times 0 \times 0 = 0 0 tells that the order of zeroes is decisive.

1 0 d x \frac10 d x = 0 with high order of 0 for d x . d x. Generalization onto 0 0 \frac00 and 1 0 \frac10 in the past may not be sufficient, that had not enable people to handle them. But d x with multiple of width for others makes the dominated areas. The thing is equal extend or magnitude of different signs makes a cancellation into zero. I think indeterminate cannot be what is meant by false here. The only thing remained is invalid or not exist for false. Letting the singularity to contribute a value of 1 for the integral, the quest must fall onto extend of finest of d x d x applied. Gender for \infty here must only come from +1 which is not -1 where 0 is neutral.

Brilliant must show a convincing example which can prove these as f a l s e false before falsifying the solution written particularly for this question. I haven't agreed with the decision with reasons given.

The significance of my conclusion would affect the consequence for any \Large ^\lim_{m \to a}\int_{-m}^{m} \frac{d x}{x^n} = a 1 n 1 n \frac{a^{1 - n}}{1 - n} - ( a ) 1 n 1 n . \frac{(-a)^{1 - n}}{1 - n}. Including those which converge at x = 0 0^- and x = 0 + 0^+ , the consequence means that all integral for 1 x n \frac{1}{x^n} with x crossing zero would only be values not exist!

For \Large ^\lim_{\epsilon \to 0}(\int_{-1}^{-\epsilon} \frac{d x}{x^n}+ \int_{\epsilon}^{1} \frac{d x}{x^n}) however, all odd functions should produce integral of zero, which is called the principal value of an integral.

0 ϵ d x x \int_{0}^{\epsilon} \dfrac{d x}{x} = ln ϵ 0 \ln | \frac{\epsilon}{0} | hinted for integral of positive infinity when leaving 0 to positive side. But the value of 1 0 \frac10 is a not exist.

Lu, the integrals that you defined do not exist, for the very reason that Issac mentioned. When we are integrating about a pole (value tends to infinity), we are taking the limit of the integrals over the interval. If this limit exists (has to be constant for all sequence of intervals converging to the interval that we want), then we say that the integral exists. In particular, the integral about poles of the form 1 x n \frac{ 1 } { x^n} do not exist for n 1 n \geq 1 , but they exist for 0 < n < 1 0 < n < 1 .

I have updated the answer to "False and False".

Calvin Lin Staff - 5 years, 5 months ago

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The significance of my or our conclusion would affect the consequence for any \Large ^\lim_{m \to a}\int_{-m}^{m} \frac{d x}{x^n} = a 1 n 1 n \Large \frac{a^{1 - n}}{1 - n} - ( a ) 1 n 1 n . \Large \frac{(-a)^{1 - n}}{1 - n}. Including those which converge at x = 0 0^- and x = 0 + 0^+ with 0 < n < 1, the consequence means that a l l i n t e g r a l f o r 1 x n w i t h x c r o s s i n g z e r o w o u l d o n l y b e v a l u e s o f n o t e x i s t ! \color{#3D99F6}{~all~integral~for~\frac{1}{x^n}~with~ x~ crossing~zero~would~only~be~values~of~not~exist!} It is quite surprising that singularity such as at x = 0, if not checked, then the integral shall not reveal a problem.

Here, we take 1 3 \Large \sqrt[3]{-1} = 1 R e a l n u m b e r s o n l y , = -1 ~ \forall~Real~numbers~only, for example.

Lu Chee Ket - 5 years, 5 months ago

We have only o n e one 0 but t w o two infinities namely -\infty and + . +\infty. I have understood what is meant by having different limits for different values. I am sure that you haven't read to understand my writing on another hand. Please read my solution and then reconsider. I have several supporters who agreed with me as well, among whom I found strong members. Taking 1 0 d x x + 0 + 1 d x x \int_{-1}^{0-} \dfrac{d x}{x} + \int_{0+}^{1} \dfrac{d x}{x} = 1 1 d x x \int_{-1}^{1} \dfrac{d x}{x} of different magnitude or different extend of zero for both sides of zero which I called ϵ \epsilon in this context is a m i s t a k e mistake . Divergence is actually not the actual fact of why a form of \infty - \infty cannot be zero; it is two different magnitudes or two different extends of infinities that makes it an indeterminate. I raised this question because I have noticed this mistake. Thanks!

Lu Chee Ket - 5 years, 5 months ago

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I have read your solution. As pointed out by Issac (and rephrased by me above),

when we are integrating about a pole, we are taking the limit of the integrals over intervals that converge to the final interval that we want.

More explicitly, I f ( x ) d x \int _I f(x) \, dx exists if any only if:
for any sequence of intervals I n I_n that converge to I I , lim I n f ( x ) d x \lim \int_{I_n} f(x) \, dx exists and is independent of the choice of intervals.

In particular, the line in your solution that is incorrect is claiming :

1 1 d x x \int_{-1}^1 \dfrac{d x}{x} = \Large^\lim_{\epsilon \to 0}[\int_{-1}^{\epsilon^-} \dfrac{d x}{x} + \int_{\epsilon^+}^1 \dfrac{d x}{x}] ]

You have chosen intervals of the form I n = [ 1 , ϵ ] [ ϵ , 1 ] I_n = [-1, -\epsilon ] \cup [\epsilon , 1 ] to evaluate a particular limit. However, you have not shown that the choice is independent of the choice of intervals . In particular, the example given by Isaac shows that this is not true. Hence, the limit is undefined.

Calvin Lin Staff - 5 years, 5 months ago

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@Calvin Lin x = 0 is an only point on the x-axis for example. Taking 0 0^- and 0 + 0^+ to replace 0 in situation like this is a conclusive approach and therefore a well known. However, I think we must not treat such a change in an improper way. The only way that suppose to be taken would be an equal and fair naturally. This differs from the case for having -\infty and + +\infty I think, because -\infty and + +\infty are not to be changed but they are just not equal in magnitude or extend naturally because they add up to 2 unidentified numbers of different magnitude arbitrarily.

I find that d x x = ln x + C \int \dfrac{d x}{x} = \ln | x |+ C is not just what we think we see. It actually means for what I have elaborated in the solution, I think. Provided this is true as the case, then it has implicitly suggested that n n d x x \int_{-n}^{n} \dfrac{d x}{x} is indeed zero rather than an indeterminate.

As an ultimate conclusion, we don't need to mention Cauchy's Principal Value wanted but should be set as a certainly preferred consequence. We can just update the world. I shifted my gear to neutral to make changes to accept your correct suggestions last time.

Lu Chee Ket - 5 years, 5 months ago

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@Lu Chee Ket When we talk about integrals, we generally talk about them as Riemann integrals or Lebesgue integrals (and both are equal on continuous functions defined on an interval). Riemann integration is developed from Riemann Sums , and it is clear why we care about "any sequence of intervals that converge to I I ".

I would prefer to stick to this convention. If you want to bring in a new system of calculation, please state that explicitly in your question. Most people write it as: P V 1 1 1 x d x = 0 PV \int_{-1} ^ 1 \frac{1}{x} \, dx = 0 , which is a true statement because you quantified which integral universe you are working with.

For example, those who want to claim that " 1 + 2 + 3 + = 1 12 1 + 2 + 3 + \ldots = - \frac{1}{12} , should make it explicit that they are looking for the zeta function regularization of the infinite sum. Otherwise, the answer would be that the sum does not converge / is infinity.

Calvin Lin Staff - 5 years, 5 months ago

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@Calvin Lin There is an immediate issue of whom to mark them right or to mark them incorrect. According to rules you explained to me several days ago, those who answered True and True which are reasonable as one possibility shall certainly be marked correct.

Nevertheless, I haven't regretted and my answer is still True and True.

Lu Chee Ket - 5 years, 5 months ago

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@Lu Chee Ket They are intentionally marked incorrect, because that is what the mathematical theory says.

Calvin Lin Staff - 5 years, 5 months ago

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@Calvin Lin Thanks for letting everyone to enjoy the experience of having an international reference of being judged. We can therefore know the answer of how the world is seeing the answer to this question. However, I believe we also strive to look for an ultimate answer via a reconsideration for any possible update. We should like to quote to ask whether what we can raise for discussions are inclusive in what is meant by mathematical theory. To make this simple, we should ask whether our suggestion which could have been missed out from consideration in the past is indeed reasonable to be accepted. Do you think you can agree with my claim that n n d x x \int_{-n}^n \dfrac{d x}{x} should be the correct thing to unite as one same thing through out to differ from two separated portions right from the beginning?

Lu Chee Ket - 5 years, 5 months ago

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@Lu Chee Ket I do not understand what you mean by "correct thing to unite as one same thing through out to differ from two separated portions right form the beginning".

From the perspective of someone studying about Riemann integrals, it is important for them to know why "things do not cancel out".

Calvin Lin Staff - 5 years, 5 months ago

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@Calvin Lin Mysterious things can become a barrier for people to know the truth when there is no way to ask or to discuss.

1 0 d x x \int_{-1}^{0^-} \dfrac{d x}{x} + 0 + 1 d x x \int_{0^+}^{1} \dfrac{d x}{x} simlilar to d x x \int_{-\infty}^{\infty} \dfrac{d x}{x} and 1 d x x \int_{-\infty}^{-1} \dfrac{d x}{x} + 1 d x x \int_{1}^{\infty} \dfrac{d x}{x} {All united as one category.}

1 1 d x x \int_{-1}^{1} \dfrac{d x}{x} similar to ^\lim_{n \to \infty} \int_{-n}^{n} \dfrac{d x}{x} and ^\lim_{n \to \infty} (\int_{-n}^{-1} \dfrac{d x}{x} + 1 n d x x ) \int_{1}^{n} \dfrac{d x}{x}) {All united as another one category.}

Things do cancel out provided we treat them correctly.

In fact, 1 1 d x x \int_{-1}^{1} \dfrac{d x}{x} = ^\lim_{\epsilon \to 0}(\int_{-1}^{\epsilon^-} \dfrac{d x}{x} + ϵ + 1 d x x ) \int_{\epsilon^+}^{1} \dfrac{d x}{x}) is the fair and correct treatment.

Adjust mind set by putting aside thought of determining the limit for 1 0 d x x \int_{-1}^{0^-} \dfrac{d x}{x} + 0 + 1 d x x \int_{0^+}^{1} \dfrac{d x}{x} for whether an integral is valid, as this is actually not suppose to be the case to imply.

(Two different extends) is not the same as (one same extend).

Lu Chee Ket - 5 years, 5 months ago

@Calvin Lin This question is like question onto 0 0 . 0^0. The thing is Brilliant is always taking the right to enforce for own opinion to be a correct answer.

Lu Chee Ket - 5 years, 5 months ago

@Calvin Lin You meant categorized under some persons' names for ease to refer for our meanings. However, when we have arrived to an ultimate conclusion, we must take the most reasonable as a preset where a mention like P V P V should not be there or otherwise it is not true strictly. Do you think that singularity of zero or an uneven 0 + 0^+ and 0 0^- contribute to actual reason for you to conclude?

Converged sequence appears to be sum for series rather than about an integral. You may claim that such a form of + × 0 + = i n d e t e r m i n a t e , +\infty \times 0^+ = indeterminate, but an integral as can be seen from graph does show that x = 0 can have no area in the limit as 0.

From some other integrals of 0 a \int_{0}^{a} converted into 0 + a \int_{0+}^{a} to be evaluated, its difference compared to integral in our discussion to cross x = 0 is not obvious. x = 0 can have no width. You may like to visualize the case with outside segments of the y-axis for x = 0 being fully occupied with areas which cannot be ignored. But we simply don't see its area.

I would rather think that consequence of P V 1 1 d x x = 0 P V \int_{-1}^1 \dfrac{d x}{x} = 0 should be taken as preferred reference for 1 1 d x x = 0. \int_{-1}^1 \dfrac{d x}{x} = 0.

Lu Chee Ket - 5 years, 5 months ago

@Calvin Lin Once you agreed to prove or disprove by changing 0 into 0- and 0+ but not in the fair way I suggested, you have actually denied divergence as its critical reason for invalidity claimed or suggested. In fact, I think we must not take template like this as a fixed reference. I don't think 1 0 \frac{1}{0} should make a zero into non-existence. This is particularly true when ln ( 1 ) \ln(-1) is not necessarily there for integrals of crossing zero that I have introduced. x = 0 is having no width for area and not affecting the integral at all in the limit where ϵ -\epsilon and ϵ \epsilon have taken the areas and neutralize!

Lu Chee Ket - 5 years, 5 months ago

@Calvin Lin I think the main point for this question is the definition or opinion of requirement of existence of sole limit allowed for a claim of an exist integral or a valid integral has actually overwritten the r i g h t right given t o to c h a n g e change 0 0 into others.

My point here is your claim can only be true for n 0 d x x \int_{-n}^{0^-} \dfrac{d x}{x} + 0 + n d x x \int_{0^+}^{n} \dfrac{d x}{x} but not n n d x x . \int_{-n}^{n} \dfrac{d x}{x}. Like current and voltage, n 0 d x x \int_{-n}^{0^-} \dfrac{d x}{x} + 0 + n d x x \int_{0^+}^{n} \dfrac{d x}{x} is comparable to d x x \int_{-\infty}^{\infty} \dfrac{d x}{x} only, whereby only n n d x x \int_{-n}^{n} \dfrac{d x}{x} is comparable to ^\lim_{n \to \infty} \int_{-n}^{n} \dfrac{d x}{x}. We have no right to change it unless we do fairly onto it.

Lu Chee Ket - 5 years, 5 months ago

I ignored this completely as I found not the point of discussion. This simply stands for d x x n = x 1 n 1 n + C \int \dfrac{d x}{x^n} = \frac{x^{1 - n}}{1 - n} + C for no singularity onto integral with x = 0 for 0 < n< 1, am I right?

Lu Chee Ket - 5 years, 5 months ago

Sir, can you give me graphical explanation to this question?

Swapnil Roge - 5 years, 5 months ago

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