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Suppose I want to integrate $f(x)+g(x)$. Can this be written as $\int f(x)+g(x)\, dx$ or are brackets necessary, i.e. $\int \left(f(x)+g(x)\right) \,dx$?

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    I fail to see how the version without parenthesis could make sense. If anybody knows references of texts using this convention, please share.2012-04-15

2 Answers 2

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No.

But be careful that in general $\int f(x) +g(x) d x \neq \int f(x) d x + \int g(x) d x.$

E.g. consider the integral over $\mathbb{R}$ with $f=1$ and $g(x)=-1$ times the indicator function of $|x| >1$.

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    Depends on your definition;) but probably yes.2012-04-15
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This is a conventional exception. If I were going to write $(f(x)+g(x))\,dx$, with no integral sign (e.g. when a differential equation is written as $a(x,y)\,dx+b(x,y)\,dy=0$ and the expression $a(x,y)$ or $b(x,y)$ has several terms) I would not omit the parentheses. Everything within the parentheses is multiplied by $dx$. If $f(x)+g(x)$ is in meters per second and $dx$ is in seconds, then $(f(x)+g(x))\,dx$. However, in something like $\displaystyle\int x^2+3x+10 \, dx$ it is quite conventional to omit delimeters. It is as if the expression $ \int \cdots\cdots\cdots\cdots dx $ acts in some way on whatever is written where "$\cdots\cdots\cdots\cdots$" appears.