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Some context: I'm an engineer, and I tend to have a rather unusual way of understanding and thinking about things, most likely related to my being autistic. I found this question on the HNQ and upon reading it I felt rather confused.

I have always understood that the indefinite and definite integrals are two sides of the same underlying concept, that there is no meaningful difference between them other than that one evaluates to a function and the other to a quantity. Essentially, the definite integral is what you get from running the numbers on the result of an indefinite integral, and saying it's unrelated to the definite integral is like saying that $sin(x)$ is fundamentally different from $sin(a)$, assuming $x$ is a variable and $a$ a constant, for no reason other than that $x$ is a variable and $a$ a constant.

To me, whether the definite and indefinite integrals are two sides of the same thing, or two closely related but different things, seems more like a question of philosophy than mathematics.

Where is the flaw in my understanding? Is there one?

The point is, there are three slightly different concepts here:

1) If $f$ and $F$ are functions with the same domain, and $F'(x)=f(x)$ on that domain, then we say that $F$ is an antiderivative of $f$.

2) If $f$ is a function on the interval $a le x le b$, then the definite integral of $f$, $int_a^b f(x) , dx$, is (loosely speaking) the area under the graph of $y=f(x)$ for $a le x le b$, or (more precisely speaking) a limit of Riemann sums. This definition does not involve any derivatives.

3) If $f$ is a function on the interval $a < x < b$ and $c$ is a point in that interval, then the function $G(x)=int_c^x f(t) , dt$ could be called an indefinite integral of $f$. This definition of the function $G$ comes directly from the definition in 2) of the definite integral, so it also does not involve any derivatives.

You are saying that you don't see any difference between 2) and 3). You are correct! If your mental definition of "indefinite integral" is 3), then an indefinite integral is just a definite integral with some unknown limits.

The question you're linking is saying that many calculus books use 1) as the definition of an "indefinite integral": that is, they say that if $F$ is an antiderivative of $f$, then $int f(x) , dx = F(x)+C$ is the indefinite integral of $f$. They can get away with this because of the fundamental theorem of calculus, which says (in part) that

$$
fracddxint_c^x f(t) , dt=f(x) , .
$$

That is, the function $G(x)$ defined in 3) is an antiderivative of the function $f$, so these two notions of "indefinite integral" are closely related.

But closely related does not mean identical! K B Dave's answer is giving you some examples of situations where the two definitions 1) and 3) diverge slightly. Specifically:

• We gave the definition 3) on an interval $a < x < b$, but it doesn't work very well on a more complicated domain. This means that definition 1) is more robust when $f$ has singularities.


• The fundamental theorem of calculus says that every "indefinite integral" in the sense of 3) is an antiderivative. It does not say that every antiderivative is an indefinite integral, which in fact is false; you can have functions $f$ and $F$ where $F'(x)=f(x)$, but there is no way to write $F(x)=int_c^x f(t) , dt$.

$DeclareMathOperatordomdom$

"Reasonable" functions $f$ can have antiderivatives $D^-1f$ that are not of the form $xmapsto int_a^xf(t)mathrmdt$ for any $a$ in the domain of $f$.

1. Let $U=(-infty,0)cup(0,infty)$, and consider
   $$beginalign
   f&: UtomathbbR &
   x&mapsto frac1xtext.
   endalign$$
   Then
   $$D^-1f(x)=begincases
   ln(-x)+C_- & x < 0\
   ln(x) + C_+ & x > 0
   endcases$$
   where $C_-$ and $C_+$ can differ—because the domain is not path-connected, there is no way to integrate from positive $x$ to negative $x$.


2. Consider
   $$beginalign
   f&:mathbbRto mathbbR &
   x&mapsto frac11+x^2text.
   endalign$$
   Then
   $$int_a^xf(t)mathrmdt=arctan x - arctan a$$
   but
   $$D^-1f(x)=arctan x + Ctext;$$
   since the magnitude of $arctan a$ is bounded by $pi/2$, if $lvert C rvert geq pi/2$ then $D^-1f(x)$ is not of the form $int_a^xf(t)mathrmdt$.

As defined, the definite integral and the indefinite integral are totally different things.

The definite integral $int_a^b f(x) dx$ is defined by looking at approximations to $f$ which are piecewise linear functions, looking at the areas of the resulting trapezoids, and then examining what happens in the limit as these approximations get more and more accurate.

The indefinite integral $int f(x) dx$ is defined by searching for a function $g$ such that $g' = f$.

As you can see, on the face of it, these two concepts appear to have nothing to do with each other whatsoever. The definite integral has nothing to do with derivatives. And the indefinite integral has nothing to do with piecewise linear approximations or areas of trapezoids.

It just so happens that the definite integral and the indefinite integral are actually closely related: given some reasonable assumptions, if $F(x) = int f(x) dx$, then $int_a^b f(x) dx = F(b) - F(a)$. So the definite integral can, in fact, be found by finding the indefinite integral and then plugging some numbers into it. Likewise, the indefinite integral can be found by finding definite integrals where one endpoint is a variable. But the definitions are totally different.

Ultimately, maybe it doesn't matter. The Dedekind real numbers and the Cauchy real numbers are defined in completely different ways, but they end up defining exactly the same thing (at least in ZFC). Definite integrals and indefinite integrals aren't exactly the same, but unless you're doing real analysis on "ill-behaved" functions, the distinction doesn't really have any consequences.

Though technically the one is a number and the other a function, you can write

$$int_a^b f(x),dx=F(b)-F(a)$$

and

$$F(x)=int_c^x f(x),dx$$ where $F$ is an antiderivative of $f$ and $c$ is an arbitrary bound. So both concepts are virtually interchangeable.

Essentially, the definite integral is what you get from running the numbers on the result of an indefinite integral, and saying it's unrelated [...No-one would claim they are "unrelated"; merely that the are "different". "different" most certainly does not mean "unrelated"...] to the definite integral is like saying that $sin(x)$ is fundamentally different from $sin(a)$, assuming $x$ is a variable and $a$ a constant, for no reason other than that $x$ is a variable and a $a$ constant.

I think this is your flaw. That $x$ is a variable and $a$ is a constant is a HUGE and very fundamental difference.

The number $0.86602540378443864676372317075294...$ can be thought of in many ways but ultimately it is "what it is". One of the things that it is is one half of the positive number that when squared is $3$. Another thing it is is $sin frac pi 3$. The number doesn't change what it is just because we describe it in different ways for different purpose.

If $x$ is a variable then $sin(x)$ is not a specific number (which $sin a$ is). $sin(x)$ is a theoretical possible unknown value of a number that would be sine of whatever number $x$ would be if it were pinned down. But as it isn't pinned down, it is an indefinite value. In a comment you claimed we don't call $sin a$ the "definite sine" and $sin x$ the "indefinite sine". We don't use those exact words but we most certainly do use those concepts and distinguish between them.

To add to the confusion the function $sin$, itself, is different than $sin(x)$. $sin = (x,y)$ ordered pairs so that $y = sin x$ is the concept of the function itself; a collection of all ordered pairs, whereas $sin(x)$ is an indefinite output value of the function for an indefinite input value, and whereas $sin (a)$ is a definite output value of the function for a definite input value.

These are three different applications of one concept or "three sides of the same coin". No-one is saying they are unrelated. But they are different things.

......

By the way, you've heard the joke about the a coworker who thinks the capital of Norway is in Sweden [a fundamentally impossible and inconsistent error; a capital of a country can't be in a different country so such an error is inconceivable for someone to make]? That's because he thinks Oslo is in Sweden [a stupid, but conceivable error] and Oslo is the capital of Norway [a fact; albeit one he is unaware of] and therefore he thinks that the capital of Norway [Oslo] is in Sweden.

This is kind of the same thing. The definite actuality of Oslo, and the indefinite concept of the capitol of a country are two different things even though they may evaluate to the same thing (if the country you choose is specifically Norway-- but not Sweden).