Take the 2-minute tour ×
Mathematics Stack Exchange is a question and answer site for people studying math at any level and professionals in related fields. It's 100% free, no registration required.

This question already has an answer here:

My friend Boris (Boryan) gave me a task, and completely refuses to give the answer what's wrong here. $$x^2=\overbrace{x+\cdots+x} ^{x\text{ times}}$$ $$(x^2)'=(x+\cdots+x)'$$ $$2x=1+\cdots+1$$ $$2x=x$$ $$2=1$$ Yeah! I've succesfully copypasted latex formulas!


I think the problem is in non-formal symbols. It brings me to question, what is the result for $(\sum_{i=1}^{x}x)' = ?$ That's usually an obstacle for those who memorised many things without clear understanding of definitions. So, I'm interested in fundamental mistake of this equations, because I want to get out of this mess)

share|improve this question

marked as duplicate by Claude Leibovici, Asaf Karagila, Grumpy Parsnip, Grigory M, Martin Sleziak Apr 19 at 13:17

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

2  
Calculus alliterates with continuous not discrete. –  Awesome Apr 18 at 6:46
    
There was a typo in previous one –  Awesome Apr 18 at 6:47
2  
Intuitively, both the summands and the length of the sum change with respect to $x$. The differentiation step here only takes account of the change in the summands. –  James Wood Apr 18 at 10:44
1  
One cannot differentiate a function that is only defined at integer values at all. –  Marc van Leeuwen Apr 18 at 11:29
3  
Note that if you also differentiate the "$x$ times" part and add it on, you get the correct answer. See this MO Post. –  ronno Apr 18 at 13:48

7 Answers 7

up vote 9 down vote accepted

The first misconception is the definition of $x$-times applied operations.
If one ignores that, there still remains the error in the derivation $$\frac d{dx}\sum_{i=1}^x x \neq \sum_{i=0}^x \frac d{dx} x = x$$ You have to use Leibniz integral rule, seeing the sum as a special type of integral, namely $$\frac d{dx} \sum_{i=1}^x x = \frac d{dx} \int_0^x x d\#(i) = x|_{i=x} \cdot 1 - x|_{i=0} \cdot 0 + \int_0^x 1 d\#(i) = x + x = 2x$$ Where $\#$ denotes the counting measure. If you understand this, I can elaborate on the RHS.

share|improve this answer
    
You added "counting measure" because x is integer? I'm not familiar with this step, could you explain it a bit? –  Ralor Apr 18 at 5:27
    
@Ralor The counting measure is a special measure defined on $(\mathbb Z, \mathcal P(\mathbb Z))$ by "counting the elements", i.e. $$\#(A) = \text{number of elements in } A$$ This uses measure theory, though. –  AlexR Apr 18 at 5:30
    
I believe it is straghtforward answer, but I still don't understand it for 100%. But now I'm sure how "measure theory" translates into Russian, so I will try to do so. –  Ralor Apr 18 at 5:51
    
@Ralor this is the english wikipedia entry. At the left you can see a list of other languages (Ukrainian was among them, for example) - maybe this will help find the appropriate term. –  AlexR Apr 18 at 5:56
    
I think you know that translations Russian -> Russian is often possible, especially when you're talking about some non-trivial things having only partial knowledges (just some of Discrete analysis in my case) –  Ralor Apr 18 at 6:03

the mistake is at the beginning $$x^2=\overbrace{x+\cdots+x} ^{x\text{ times}}$$ fails miserably when $x$ is not a natural number as the expression $$\overbrace{x+\cdots+x} ^{x\text{ times}}$$ is meaningless for example in case $x=1.37$ what does $$\overbrace{1.37+\cdots+1.37} ^{1.37\text{ times}}$$ mean?

share|improve this answer
    
I didn't thinked about it 46 mins ago. It's really important to define $D(X)$ at first. But I can't upvote right now) –  Ralor Apr 18 at 5:55
    
It's precisely as meaningless as fractional-order differention or fractional order Fourier transformation. That is, none -- all of these operations are well defined. The one you propose is a complicated way of writing 1.8769. Just because you're used to something only for integer arguments doesn't mean it doesn't extend (unambiguously). –  Eric Towers Apr 19 at 0:00
    
@EricTowers thank you Eric you are correct, i should rephrase, $$\overbrace{1.37+\cdots+1.37} ^{1.37\text{ times}}$$ is utterly meaningless unless a proper extension is defined [unless you can prove there is ONLY one such extension possible..].. for real numbers.. as it stands i still see the first line as the problem as it is not well defined, i did not say it is not well definable by some extension.. i welcome your comments.. thx –  userX Apr 19 at 4:26

If $x$ is any real positive number then the equality should be written in the form $x^2=\int_0^xxdy$ which is evidently true.

Then the derivatives are $(x^2)'=2x$ and $\left( x\int_0^x dy\right)'=\int_0^xdy+x=x+x=2x$

share|improve this answer

The problem is that $(x + \cdots + x)' \not= 1 + \cdots +1$

share|improve this answer
5  
Well it is, but not if the amount of x's is variable too. –  Jori Apr 18 at 8:30

$$(x^2)'=\bigg(\sum_1^xx\bigg)'=\sum_1^xx'+\sum_1^{x'}x=\sum_1^x1+\sum_1^1x=x+x=2x.$$

share|improve this answer

Visualize the RHS graph. You will see that it is discontinuous. So can't differentiate. If you take $x^3$ instead of $x^2$, yon can also prove 1=2=3! Surely try that too!

share|improve this answer

You have a genuine result that $(f+g)' = f' + g'$.

It follows from this that you can do the same for any finite sum of functions.

But now you seek to do the same for sums with a variable number of terms according to the value of $x$. That's not an application of the rule you have, because you're no longer looking at a function that's a sum of finitely many functions. Rather you're looking at a function whose value at a given point has been written as a sum of finitely many values. But so what if it is? Every real number is the sum of finitely many real numbers, and for that matter every integer is the sum of finitely many 1s! This tells us nothing about the function and so we shouldn't expect it to tell us anything about the derivative.

It's also a problem of course trying to differentiate a function of integers in the first place, and if $x$ isn't an integer then the RHS makes no sense. But you could overcome that objection by choosing instead to differentiate $x$ floor($x$) as the same sum with floor($x$) terms. The answer would still be wrong, indeed the function isn't differentiable or even continuous at integer values of $x$. For any of the piecewise continuous and differentiable regions between the integers, within which the value of floor($x$) is a constant, it would now give you the correct derivative, floor($x$).

share|improve this answer

Not the answer you're looking for? Browse other questions tagged or ask your own question.