# What is this notation called? [closed]

$$1 + 2 + \cdots + n = \frac{n(n+1)}{2}.$$ I don't need an explanation of the formula. I just want a reference for what the symbols mean.

• Can you clarify which are the symbols which are stumping you? Possibly, the $\ldots$? – астон вілла олоф мэллбэрг Feb 19 '17 at 3:54
• I don't see the problem with this question. When you use $\Sigma$, that's called "summation notation." The OP is asking what it's called when you use suspension points. Unfortunately, I don't know the answer, but it seems plausible that there's accepted terminology for this. Neither of the two answers given so far attempts to answer the question in the title. – user49640 Feb 19 '17 at 4:04
• The symbol $+$ stands for "plus," the symbol $=$ stands for "equals." – Joel Reyes Noche Feb 19 '17 at 4:04
• @crazyfox : Which symbols, specifically? – MPW Feb 19 '17 at 4:35
• @ MPW I want to know what ALL the symbols mean. I can generally infer what the equation represents, but I want a reference to look these symbols up. I don't need you to tell me what a few mean, when I would just run into more and not know them. Is there a standardized way of using this? Or is it just ad-hoc and dependent on the author's intent? – crazyfox Feb 19 '17 at 4:45

It sounds like you are just having difficulty interpreting the equation because of an error in MathJax rendering on your side.

To us, the equation displays as

$$1 + 2 + \cdots + n = \frac{n(n+1)}{2}$$

Screengrabbing this and rendering it as a picture this is:

Assuming MathJax is rendering correctly, the two above look identical.

In ascii, this could be written as 1 + 2 + ... + n = n(n+1)/2

In words, this is "The sum of the first n natural numbers is equal to the expression n times the number one larger than n all divided by two."

For information on how to type in MathJax and $\LaTeX$, again visit this page. For information on how to get it to correctly render, try using a different or more up to date browser. If still having difficulty, try asking on Meta for more help.

• Okay that is very clear, thanks. For future reference, why was this question downvoted? Is it just assumed that everyone knows what Mathjax is? – crazyfox Feb 19 '17 at 5:15
• It is more than likely that it was downvoted because people could not understand the true nature of your question. It was incredibly unclear that what you were having difficulty understanding was the use of the dollar signs and other MathJax formatting and as far as we could tell you even didn't know what a plus sign was. A question so introductory as what a plus sign is is not going to be in the scope of problems asked on this site. A question about MathJax formatting needs to be made clear that it is the focus of the actual question. Use more descriptions in your questions in the future – JMoravitz Feb 19 '17 at 5:18
• @crazyfox: Everyone else just sees the equation as shown in the picture in this answer, and didn't know that you were seeing something different. – Eric Wofsey Feb 19 '17 at 5:26

It is saying that the sum $1$+ $2$ + $3$+,,, all the way to plus $n$ is equal to half of $n(n+1)$

For example,

$$1+2+3+...+98+99+100=\frac{100(101)}{2}=5500$$

• What do the '$' signify? Do you just wrap everything in '$'? If so, why not just omit them? What is '\frac'? Is it applied to the next two terms to make a fraction? Why not just omit the '\'? What is the difference between braces and parens? I have seen several equations in CompSci that are pretty easy concepts for me, some I have even used in practice, but when this notation is used I am just not sure what anything means. I just want to know what the name of this notation is so I can use a reference. I am not comfortable with $1$ representing the value 1 unless I know why the \$ are there. – crazyfox Feb 19 '17 at 4:37