# Which is the best English term for "the result of a mathematical integration"?

Mathematics Asked by Jundan Luo on November 10, 2020

I am writing an academic paper. And I wonder which is the best English term for "the result of a mathematical integration".

For example, I have a mathematical integration as below.

$$F = int f(u) du$$

Which is a more suitable name for $$F$$? "integral $$f(u)$$" or "integrated $$f(u)$$"? I must name $$F$$ in a way expressing its relationship with $$f(u)$$.

Thanks for your comments! As it is the "integral of $$f(u)$$", can I name it "integral $$f(u)$$" for short? I think I have to omit the preposition when defining a new academic word. ($$f(u)$$ is an already-defined physical term.)

The notation $$int f$$ is generally used in one of two contexts: either it represents a number (which we generally interpret as the area under the graph of $$f$$, but it is actually more general than that), or it represents a function.

### A number

If $$int f$$ represents a number, then it is a definite integral. More commonly, this is written in the form $$int_{a}^{b} f(x) ,mathrm{d}x qquadtext{or}qquad int_E f(x) ,mathrm{d}x.$$ Sometimes, a definite integral will be written without giving explicit bounds for the integration, or without specifying a domain of integration. In such a context where a number is still meant, this notation is generally understood to indicate integration over the entire domain of the function. For example, $$int mathrm{e}^{-x^2},mathrm{d}x = int_{-infty}^{infty} mathrm{e}^{-x^2}, mathrm{d}x = sqrt{pi}.$$ In this context, the number which is obtained by integration is called the integral. More precisely, one might say that $$int_E f(x),mathrm{d}x$$ is called the "definite integral of $$f$$ over $$E$$".

### A function

The same notation is also used to denote a function. For example, if $$f$$ is a "sufficiently nice" read-valued function defined on the real numbers, and there is a function $$F$$ with the property that $$F'(x) = f(x)$$ for all $$x$$, then we say that $$F$$ is an antiderivative or primitive of $$f$$. This is often written as $$F(x) = int_{a}^{t} f(t),mathrm{d}t qquadtext{or}qquad F = int f.$$ It should be noted that $$F$$ is not unique—a function may have many antiderivatives, though these antiderivatives differ by only a constant, so it is not hard to specify the entire family of antiderivatives. The notation $$int f$$ is, perhaps, confusing, but it is justified by the Fundamental Theorem of Calculus, which demonstrates that definite integrals and antiderivatives are related, e.g. in the setting of Riemann integration, $$F$$ is an antiderivative of $$f$$, then $$int_{a}^{b} f(x),mathrm{d}x = F(b) - F(a).$$ This function might also be called the indefinite integral or inverse derivative. Wikipedia gives another couple of terms.

### A note on notation

The notation $$f$$ denotes a function, while $$f(u)$$ or $$f(x)$$ denotes a value of that function—this latter notation represents a number (or dependent variable), not a function. Thus it is a little strange to talk about "the integral of $$f(u)$$". Thus it is fine to say that something is the "integral of $$f$$", but it is not quite right to say that it is the "integral of $$f(u)$$".

### TL;DR

If $$int f$$ denotes a number, that number is the integral or definite integral of $$f$$ (over some interval or domain). If $$int f$$ denotes a function, that function is an antiderivative or primitive of $$f$$. In either case, I would not elide the preposition—keep that "of" in there.

Answered by Xander Henderson on November 10, 2020

I agree with some comments. The term "integral" is used both for the problem and for the answer. This is like a lot of other words in mathematics:

5+3 is an easy sum
What is 5+3? The sum is 8.

5! is the product of the numbers from 1 to 5
120 is the product of the numbers from 1 to 5

Answered by GEdgar on November 10, 2020

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