Tips on writing a formula in rigorous way

[serbring]

Dear,

I am writing a report of a tool I have developed where there is a chapter with all the formulas included in the tool. I have a formula like the following:

$$P(l,1)=\sum_k PF(k,l)$$

but P(l,1) contains the sum of all the positive PFs, while P(l,2) contains the sum of all the negative PFs. Is there any elegant/rigorous way to write the formula?

thanks best regards

 

[mfb]

With the Heaviside function: $$P(l,1)=\sum_k PF(k,l) \Theta(PF(k,l))$$
A bit ugly but compact:$$P(l,1)=\sum_{k,PF(k,l)>0} PF(k,l)$$
Maybe like this:$$P(l,1)=\sum_k \begin{cases} PF(k,l) &\mbox{if } PF(k,l)> 0 \\
0 & \mbox{else } \end{cases} $$
If you need this type of sum often, you could introduce it and then shorten it to $$P(l,1)=\sum_k PF(k,l)\: \mbox {where}\: PF(k,l)>0$$

 

[serbring]

Wow! How many options! Thanks

The first one is very elegant, I like it, but not fully clear for everyone. I would go for the last one!