Indefinite Integral.

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We define Indefinite integral of a function $f$ as $\int f(x)dx$ which is the collection of anti derivatives of the function $f.$ But in many books it is written that for an integrable function $f$on $[a,b],$ $\int_{a}^{x}f(x)dx$ is the indefinite integral of $f.$ What is the difference between the two? Even we know that $\int_{a}^{x}f(x)dx$ may not be an anti derivative of $f$ for counterexample we may take $f$ as the thomae function on $[a,b]$. Then $\int_{a}^{x}f(x)dx=0 $ and so $\frac{d}{dx}\int_{a}^{x}f(x)dx=0$ $\neq f(x).$ Please help me. Thanks in advance.

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That's(definition of indefinite integral) just abuse of notation, nothing more. Those are two different concepts. Just make sure in what sense the author of talking.

The difference is, if indefinite integral exists in the first sense(in the order you kept them in your question) then indefinite integral will exist in the second sense, but if indefinite integral exists in the second sense then indefinite integral may not exist in the first sense(as your example tells us).