# Notation for every odd integer number

I have this equation:

$$f(x)=\tan(x)$$

I found the vertical asymptotes to be:

$$x=\frac{\pi}{2}k$$

What is the proper notation for that k is equal to every odd number integer(negative,positive, and zero)?

$$k\in\mathbb{Z}$$ is for every integer, but is there such a symbol for every odd number integer?

Natural numbers are positive, and sometimes zero counting numbers, my question is about integers not natural numbers.

• the vertical asymptotes are the zeros of the function $\cos(x)$ Sep 4, 2016 at 15:52
• "the vertical asymptotes are the zeros of the function cos(x)"... which are $\pi/2(2k + 1)$... I don't at all see the point of this comment. Sep 4, 2016 at 15:54
• Sidenote; where writing math you don't have to use fancy symbols for everything. There is nothing wrong (and imo it should be encouraged) to use words instead wherever possible. Sep 4, 2016 at 16:03
• You could simply write $k\rm~odd$, also Sep 4, 2016 at 16:25
• @Winther One of the most remarkable things in the history of mathematics is that we have stopped using words for everything. Having convenient notation is very important. Writing has its advantages (I prefer "for all" to $\forall$, for example), but, nevertheless, in my opinion we do need simple notation for the set of odd and even integers. $\mathbb{Z}_{2k + 1}$ is my proposal. Ahmed's idea is great as well. Sep 4, 2016 at 19:23

they are $$(2k+1)\cdot \frac{\pi}{2}$$ with $$k \in \mathbb{Z}$$

• Thanks that deserves to be a high school math hack. Sep 4, 2016 at 15:57
• @Sigma6RPU What's a "high school math hack"?
– JiK
Sep 4, 2016 at 16:38
• @JiK Anything that makes something complex, expressed in a simple elegant form. Sep 4, 2016 at 20:07
• @Sigma6RPU I would venture to say that every serious mathematician in every field of mathematics strives to express complex things in simple, elegant forms. That being the case, it does not seem very fitting to describe the natural, commonplace behavior of every mathematician as "[high school math hacking.]" I would have thought any hacking that a high school student would do would be associated with something negative. Like expressing something simple in a complicated way or something. But if it strikes you as positive, maybe you agree and it's just a difference in interpretation. Sep 6, 2016 at 19:40

You can go with $2\mathbb Z +1$

• A good option. Relatedly $2\mathbb Z$ for even and $\mathbb{Z}\setminus 2\mathbb Z$ as alternative for odd.
– quid
Sep 4, 2016 at 15:55
• @quid Isn't that quotient the set of integers mod 2? Mar 30 at 19:38
• @user3146 $\backslash \neq /$, but yes that notation is probably not ideal. Mar 30 at 19:39
• @Asinomás lol good point, I'm dumb Mar 30 at 19:41
• @user3146 so am I , I definitely see myself falling for that. Mar 30 at 19:45

As long as we're considering alternatives, you could always write $$k\equiv 1\pmod 2$$

Usually people write:

$$\frac{\pi}{2}(2k+1), k \in \mathbb{Z}$$

Sometimes people would use $\mathbb{O}$ for the set of all odd integers, but because it is not so standard they will tell you ahead of time:

$$\mathbb{O}=\{ 2n+1 : n \in \mathbb{Z}\}$$

So then, after defining $\mathbb{O}$, you would say:

$$\frac{\pi}{2}k, k \in \mathbb{O}$$

Get used the $\in$, it simply means "is a member of" some set.

• Is that an "Oh" or a zero? Sep 4, 2016 at 15:57
• $0$ as the set of all numbers? That's not a good idea, I wouldn't like to write $0 \in 0$ anywhere. Sep 4, 2016 at 15:57
• Thanks @fleablood I was wondering why it went bold. Sep 4, 2016 at 15:57
• Never seen this, but I like it. Sep 4, 2016 at 17:23
• Actually $\mathbb O$ is normally reserved for the octonian, as are $\mathbb H, \mathbb C, \mathbb R$.
– user99914
Sep 6, 2016 at 2:10

Alternatively, you could write

$$x = \frac{\pi}{2}k \quad , \quad k = \pm1, \pm3, \pm5 \dots$$

You could do $$x \epsilon \pi \mathbb{Z} / 2 \pi \mathbb{Z}$$, without resorting to $$k$$.