# Tag Info

18

This is more of an extended comment than an answer, but with regards to typesetting in $\LaTeX$, let me point out that typing \mathrm{d} should not take longer than typing d, as you shouldn't be doing either throughout your paper! The semantically correct thing to do is to define a macro representing your desired differential operator, for example, ...

18

I think the historical reason for the confusion stems from graphing trigonometric functions in polar form versus rectangular form. In rectangular form, the following statement below is true. $$\theta \quad =\quad x$$ (Where the meaning of this equality is that we let the measure of the angle on the unit circle be representative of the rectangular distance ...

17

$f^{-1}(A)$ is the preimage of the set $A$, it exists even if $f$ is not invertible. For $f: X\to Y$, it is defined as $$f^{-1}(A) = \{x\in X: f(x)\in A\}.$$ The notation is slightly confusing, I would say, but one gets used to it. It isn't really that bad of a notation, since, if $f$ is actually invertible and its inverse is $g$, then $g(A) = f^{-1}(A)$ ...

15

There is a $\theta$ on the right hand side. The definition of $\sin(\theta)$ is not just $y/r$; instead it is something like: $y/r$, after you've drawn a right triangle with $\theta$ as an angle, and where $y$ is the length of the side opposite $\theta$ and $r$ is the length of the hypotenuse. As you can see, that full definition does in fact contain a ...

11

I guess it depends on how you define $\sin\theta$. One possible definition is $f(\theta)\equiv \theta- \frac{\theta^3}{3!}+\frac{\theta^5}{5!}...$ This series converges for all $\theta$ and is called $\sin\theta$. For more info look up Maclaurin series. More generally I think you are confused about what a function is. The technical definition is daunting ...

11

Many excellent journals and books use $d$ in the italics form, such as the Journal of the American Mathematical Society (e.g., recent article by Terence Tao), London Mathematical Society Proceedings (e.g., equations 74 and 75 of this recent paper) and Spivak's Calculus. Given that reference quality publications use $d$--and that it is faster and cleaner to ...

10

If $A$ and $B$ are modules over a ring, their direct product $A \times B$ and their tensor product $A \otimes B$ are different things, so it would be unhelpful to use the same notation for them.

10

$f^{-1}$ here does not mean the inverse of $f$. It is a horrifically bad overuse of notation and can be very misleading. $f^{-1}(A)$ where $A$ is a set is the pre-image, i.e. $$f^{-1}(A) = \{x\in X: f(x) \in A\}.$$ The pre-image always exists. If $f$ were invertible, then this would coincide with what you think; however the pre-image takes care of the ...

9

These symbols have different meanings in different contexts. For instance, if we are talking about vector spaces then saying $V=U+W$ is different from $V=U\oplus W$

8

Quick answer: there is a standard to follow. Longer answer: while physicists write differential operators in upright fonts (because they follow the standards), mathematicians tend to typeset differential operators as variables (because we are lazy). I am joking, but it should be clear that $dx$ is not $d \cdot x$, and that $d$ is essentially an operator: ...

8

Looking at your profile, you post a lot on StackOverflow, so maybe a programming analogy will help. Say you want to make a Point class. There's a lot of ways you could do it. You could have member variables p.x and p.y, for the $x$ and $y$ coordinates (duh). That's the most common way. But in theory, you could also write it in terms of p.rad and p.theta. ...

5

It depends on how you want to define $\sin$ and $\cos$. I suspect you're looking for an explicit definition in terms of things you already know (such as polynomials), in which case @Karl's answer (the MacLaurin series definition) is what you're looking for. However, others like me find it more elegant to define them implicitly as bases for the the set of ...

5

Yes $\sin$ is just a function on the real (or complex) numbers. People often write $\sin(\theta)$ or $\sin\theta$ because the argument of the $\sin$ function is often an angle in physical applications, and $\theta$ is often used to denote angles. For your follow-up: $\sin^{−1}$ is the inverse function of the $\sin$ function. Similar to how $\log$ is the ...

4

There is no difference between them. It merely comes as a result of a choice in LaTeX formatting; specifically, some people write "\text{d}" (or some equivalent) for the upright formatting, but many other people don't do this for the sake of speed, and instead just write "d".

4

The notation is by no means "standard", but based on the context you provided it might mean: "$R(x,y)$ is a rational function in its arguments $x,y$" That is, $$R(x,y) = \frac{p(x,y)}{q(x,y)}$$ where $p$ and $q$ are polynomials in $x$ and $y$. In the examples you gave, the functions $g(\sin(x),\cos(x)),i(\sin(x),\cos(x))$, and $j(\sin(x),\cos(x))$ are ...

4

$\triangle ABC \sim \triangle DEF$.

3

For any function $f : \mathbb R \rightarrow \mathbb R$, the set $f^{-1}( a, \infty)$ always exists, and it is defined by $$\{ x: \in \mathbb R : f(x) > a \},$$ although it is not always measurable.

3

Just so that everyone knows what we are talking about here, let me rephrase in more familiar notation. Suppose $(\Omega, \mathcal{F}, P)$ is a probability space, and $(M, \mathcal{M})$ is a measurable space. If $X : \Omega \to M$ is a random variable (i.e. a $(\mathcal{F}, \mathcal{M})$-measurable function), it induces a pushforward measure on $(M, ... 3 The only difference it that it sometimes unclear if you consider$0$as an element of$\mathbb{N}$so that $$\mathbb{N}=\{0,1,2,...\}$$ or that $$\mathbb{N}=\{1,2,...\}$$ The first notation removes this ambiguity and makes things more clear. At the end - I would say that its a matter of preference and convention, I have seen both used many times ... 3 If you define$\Bbb N = \{1,2,3,\dots\}$, then yes: the two sets you've defined are identical, and describe the same infinite union. Note that some define$\Bbb N = \{0,1,2,3,\dots\}$3 Opinions on this issue differ, but I strongly believe that a basis (particularly in finite-dimensional linear algebra) should be a list, not a set. Here I am using "list" to mean the same thing as "ordered set". Here are two reasons why using sets does not work well: It is often convenient to talk about the matrix of a linear map$T \colon V \to W$with ... 2 If you wish to think about it from the perspective of "why isn't$\theta$on the right hand side," it may be helpful to approach sin and cos backwards - start with$\sin^{-1}(x)$and work our way backwards. Why? Because doing so makes it look more like a typical definition of a variable, with a variable on one side and some expression on the other side. ... 2 In Germany, there is the DIN 1338 standard, according to which the differntial operator d, as, e.g., e for the Euler number, should be typeset as an upright letter. According to Wikipedia, these letters are typeset in italic if AMS conventions are used. 2 By logic I think that is a good idea differentiate the symbol with the roman notation but there isnt a "standard", you can use any of them. In the same sense doesnt exist any kind of "standard" mathematical notation. I read a lot of books of many mathematical topics, everyone with different notations, not only just the infinitesimal symbol. The problem, ... 2 Unless you provide us with more context, it seems that$R$is just some function with$R:\mathbb{R}^2 \to \mathbb{R}$. What about this suggests it's a ratio? 2 It means$P$is a divisor of$a$. 1 P|a means P divides a.For P|a we can also write this as Pc=a where c is a constant.It simply means that $$P*c=a$$ or P is a factor of a. 1 If you only use this type of set, then this is impossible, because sets have no order, as you said. But you can use other objects, which are often helpful: use tupels (or vectors, which are basically the same). also, you can instead use$A$as a function:$A : \{1,2,3,4\} \to \mathbb R, \, A(k) = k$. then you can freely "access" the second element. 1 The symbol for denoting similar triangles is ($\color{blue}{\sim}$) Notice, suppose$\triangle ABC$&$\triangle PQR$are similar then in LaTex it is written as$\text{"\triangle ABC \sim \triangle PQR"}$surrounded in-between by 2 or 4 dollar signs which appears as follows $$\color{blue}{\triangle ABC \sim \triangle PQR}$$ 1 a)$1/999 = 0.001001001\ldots = 0.\overline{001}$Either of these is fairly standard notation. The overline format$0.\overline{001}$is a little more explicit, so I think it would be preferred. b)$.001 + .000001 + .000000001 + \cdots$This denotes an infinite sequence in the way that$1, 2, 3, \ldots\$ indicates an infinite sequence: a little ...

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