# Dx before the f(x) in integrals

## Main Question or Discussion Point

Why do physicists like to write ##\int dx f(x)## instead of ##\int f(x) dx##? And also when did that start?

Demystifier

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fresh_42
Mentor
Why do physicists like to write ##\int dx f(x)## instead of ##\int f(x) dx##? And also when did that start?
It's a matter of taste. E.g. if we have especially long integrands with multiple variables and constants, it can be very helpful to denote the integration variable first. My guess is, that some physicists started with it for exactly this reason: write down the "unnecessary" first and concentrate on the essential part.

Demystifier and symbolipoint
PeroK
Homework Helper
Gold Member
Why do physicists like to write ##\int dx f(x)## instead of ##\int f(x) dx##? And also when did that start?
In QM Dirac notation, it is a more natural extension of the representation of vector/state for a countable basis, ##|n \rangle##:

$$|\alpha \rangle = \sum_n |n \rangle \langle n | \alpha \rangle$$
For a continuous basis, ##|x \rangle##, this becomes:
$$|\alpha \rangle = \int dx |x \rangle \langle x| \alpha \rangle$$
Where we have the identity:
$$\sum_n |n \rangle \langle n | = I$$
and
$$\int dx |x \rangle \langle x| = I$$

Stephen Tashi and Demystifier
ZapperZ
Staff Emeritus
Why do physicists like to write ##\int dx f(x)## instead of ##\int f(x) dx##?
Says who? I'm a physicist, and I don't write it like that most of the time. I've seen mathematicians wrote dx first many times.

And why does this even merit a question? Isn't this no different than asking why someone prefers the color yellow instead of blue?

Zz.

In QM Dirac notation, it is a more natural extension of the representation of vector/state for a countable basis, ##|n \rangle##:

$$|\alpha \rangle = \sum_n |n \rangle \langle n | \alpha \rangle$$
Yes, this is the same. Mathematicians wouldn't put basis vectors first and the numbers second. It would be ##\sum a_i\vec{e}_i##, not ##\sum\vec{e}_i a_i##.

Demystifier
Says who? I'm a physicist, and I don't write it like that most of the time. I've seen mathematicians wrote dx first many times.
Ok, I thought it was obvious that I meant some physicists not all. But you are right I was sloppy.

And why does this even merit a question? Isn't this no different than asking why someone prefers the color yellow instead of blue?
Well, at some point someone must have started doing it. I might be wrong but I think the other notation have been around for a while before some, not all, started using the differential first notation. So there must have been a reason.

Also I have never seen seen it in introductory calculus/analysis textbooks. My guess is that physicists are exposed to it later on. It is puzzling to me that it is used.

ZapperZ
Staff Emeritus
Well, at some point someone must have started doing it. I might be wrong but I think the other notation have been around for a while before some, not all, started using the differential first notation. So there must have been a reason.

Also I have never seen seen it in introductory calculus/analysis textbooks. My guess is that physicists are exposed to it later on. It is puzzling to me that it is used.
Again, why is this "puzzling"? Unless you think that these things do not commute, does it matter that I write the product of A and B as BA instead of AB?

It is also a matter of typesetting style. Maybe some publishers or journals have a standard where the integration variables are written first. This is often useful if the integrand itself is a long, complicated function.

Once again, isn't this a matter of personal preference? Should I need to conform to liking the same color as you do?

Zz.

fresh_42
PeroK
Homework Helper
Gold Member
Yes, this is the same. Mathematicians wouldn't put basis vectors first and the numbers second. It would be ##\sum a_i\vec{e}_i##, not ##\sum\vec{e}_i a_i##.
Yes, exactly, and the Dirac notation doesn't quite work the mathematician's way round! It's the same with the inner product being linear in the second argument, rather than the first.

Yes, exactly, and the Dirac notation doesn't quite work the mathematician's way round! It's the same with the inner product being linear in the second argument, rather than the first.
Yes, and it is convenient that way. For the integral it is not, at least not any more. Are you saying that it is because of Dirac's notations? If you've been writing your integrals one way for a while there has to be a good reason to change later on, no?

PeroK
Homework Helper
Gold Member
Yes, and it is convenient that way. For the integral it is not, at least not any more. Are you saying that it is because of Dirac's notations? If you've been writing your integrals one way for a while there has to be a good reason to change later on, no?
All I know is that I started doing the integrals the other way round when I was learning QM Dirac notation. I've no idea whether Dirac started it. That's the only time I do it that way. Any other time I put the ##dx## at the end.

It's the same with vectors, it's only in QM that I write things back to front.

I have given it a little more thought. Here is a speculation. For some people, at least for me, the notation ##\int \dots dx## serves as parentheses to enclose the expression to be integrated, so it is natural to put it that way. Also if one does more abstract integration, or integration on groups it would seem awkward otherwise. Of course just ##\int\dots## would be fine. For other people it may be more important, at least in some cases, or more natural to think of the integral as an operator. It takes a function and it produces something else, a number, a function so on. Then it is more natural to write it as an operator ##\int dx\dots## acting on functions.

Demystifier
Says who? I'm a physicist, and I don't write it like that most of the time. I've seen mathematicians wrote dx first many times.

And why does this even merit a question? Isn't this no different than asking why someone prefers the color yellow instead of blue?

Zz.
Of course it "merits" a question. It elicited several sensible answers.

symbolipoint
Demystifier
Gold Member
By analogy with ##\sum_n f_n## I would propose to use a third notation
$$\int_x f(x)$$

PeroK
fresh_42
Mentor
By analogy with ##\sum_n f_n## I would propose to use a third notation
$$\int_x f(x)$$
Makes sense, and before there will be complaints about the location for the boundaries:
$$\int_{x \in (a,\infty]} f(x)$$

Demystifier
Gold Member
Makes sense, and before there will be complaints about the location for the boundaries:
$$\int_{x \in (a,\infty]} f(x)$$
To make the analogy with sums complete, how about
$$\int_{x =a}^{b} f(x) \;\;?$$

Demystifier
Gold Member
Why do physicists like to write ##\int dx f(x)## instead of ##\int f(x) dx##? And also when did that start?
If you have a double integral, then the notation
$$\int_{a}^{b}\int_{c}^{d}f(x,y)dxdy$$
is ambiguous, because it is not clear whether ##x\in[a,b]## or ##x\in[c,d]##. With notation
$$\int_{a}^{b}dx\int_{c}^{d}dy \,f(x,y)$$
there is no risk for such a confusion.

jedishrfu and nrqed
fresh_42
Mentor
To make the analogy with sums complete, how about
$$\int_{x =a}^{b} f(x) \;\;?$$
I guess, because I like to write the sums as well as sum over instead of from to.

And if the integration order is arbitrary, we can even write
$$\int_{x_1}^{x_2}\, \int_{y_1}^{y_2} \, \int_{z_1}^{z_2} \,f(x,y,z) = \iiint_\stackrel{\stackrel{x \in [x_1,x_2]}{y \in [y_1,y_2]}}{\stackrel{z \in [z_1,z_2]}{}}\,f(x,y,z) = \int_{(x,y,z)\in [x_1,x_2]\times [y_1,y_2] \times [z_1,z_2]}\,f(x,y,z)$$
in which case the term volume gets a complete new feeling!

Demystifier
Demystifier
Gold Member
By analogy with ##\sum_n f_n## I would propose to use a third notation
$$\int_x f(x)$$
Let me also note that Schiff in his quantum mechanics textbook uses the same notation
$${\large\sf S}_k f_k$$
for both sums and integrals.

Demystifier
Gold Member

And if the integration order is arbitrary, we can even write
$$\int_{x_1}^{x_2}\, \int_{y_1}^{y_2} \, \int_{z_1}^{z_2} \,f(x,y,z) = \iiint_\stackrel{\stackrel{x \in [x_1,x_2]}{y \in [y_1,y_2]}}{\stackrel{z \in [z_1,z_2]}{}}\,f(x,y,z) = \int_{(x,y,z)\in [x_1,x_2]\times [y_1,y_2] \times [z_1,z_2]}\,f(x,y,z)$$
in which case the term volume gets a complete new feeling!
Isn't that a proof that physicist's notation is better?

fresh_42
Mentor
Isn't that a proof that physicist's notation is better?
I might have agreed, if it wouldn't have happened, that I read this thread here in parallel

Demystifier
Gold Member
I might have agreed, if it wouldn't have happened, that I read this thread here in parallel

fresh_42
Mentor
• You cannot take away a loved infinitesimal from physicists.
• Half of them would immediately lose the ability to perform a correct substitution.

Demystifier
Demystifier
$$\int d^4x \sum_{\mu=0}^3 j_{\mu}(x)A^{\mu}(x)$$
$$j_kA^k$$