# Division using index notation

1. Sep 21, 2012

### sinad

Hello everyone,

Recently I started to use index notation, but still the division is not clear for me. I'll mention just some simple examples that I'm not sure about:

Does $a =\frac{1}{b_i}$ mean that $a = \sum_{i=1}^{3}\frac{1}{b_i}$ or $a = 1 / \sum_{i = 1}^{3}b_i$ ?

Similarly, does $a_i =\frac{b_i}{c_{jj}}$ mean that $a_i = \sum_{j=1}^{3}\frac{b_i}{c_{jj}}$ or $a = b_i / \sum_{j = 1}^{3}c_{jj}$ ?

thanks beforehand!

2. Sep 21, 2012

### Muphrid

Generally speaking, there is no summation involved if an index is not repeated on the same side of an equation. An index that is "free" (not repeated) should be free on both sides of the equation. Hence, $a = 1/b_i$ is a nonsensical expression.

$a_i = b_i/c_{jj} = \sum_j b_i/c_{jj}$ is fine, however. Divisions don't come up very often with vector quantities, though.

3. Sep 21, 2012

### sinad

Indeed, I'm sorry, what I wanted to write is $a = 1/b_{ii}$

thanks! it is clear now.

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