As a non-native speaker of English I help my friends with translating various articles that they publish abroad in English journals.
My current foray is in the world of Maths and I have a big request from English native speakers or, even better, English native speakers that are teachers of Maths or Economics.
It's nothing major, I just am not sure about a silly thing:
the definite article 
.
Let me extrapolate: do you say....
system (1) ---or--- the system (1)
figure (1) ---or--- the figure (1)
equation x=a+b ---or--- the equation x=a+b?
A context would be:
---This paper studies (the???) equation x=a+b----
Since I'm not a native speaker, my instinct is to go for the variant without the definite article but then I'm sure.
I'd be grateful for any help and nudge in the right direction.
