1. ## Quick question about notation

What does this mean...

(2)
(n)

where 2 an 2 are actually all n one big bracket. There is a "Sum of" sign before it.

Thank you!

2. Originally Posted by angels_symphony
What does this mean...

(2)
(n)

where 2 an 2 are actually all n one big bracket. There is a "Sum of" sign before it.

Thank you!
Do you mean like this?

$\displaystyle \sum {2\choose n}$

Where there upper or lower limits to the sum?

3. hmm yea it's sepposed to look like that! So..tose ae the upper and lower limits to the sum then?

And thank you!

4. Originally Posted by angels_symphony
hmm yea it's sepposed to look like that! So..tose ae the upper and lower limits to the sum then?

And thank you!
no, you write the upper and lower limits of a sum on the summation sign itself, like, for example, you would write $\displaystyle \sum_{n = 1}^N a_n$ 1 and N are the lower and upper limits respectively of the sum

Now, the notation $\displaystyle {n \choose k} = C(n,k) = _nC_k = C_k^n$ means the combination as in the combinatoric formula. it is defined as follows:

$\displaystyle {n \choose k} = \frac {n!}{k!(n - k)!}$

see here

so, for example, $\displaystyle \sum_{n = 0}^2 {2 \choose n} = {2 \choose 0} + {2 \choose 1} + {2 \choose 2} = 1 + 2 + 1 = 4$ .......here the upper and lower limits are 2 and 0 respectively