What changes with the introduction of decimal calculation in cost of manteinance?
If i have for example a city mainteinance of 3,80 will it be considered a mainteinance of 3 or 4?
I have seen also negative number about city mainteinance (i.e. capital), will this number be detracted from the total mainteinance expenses or it is considered like a 0 maintenance?
I have seen also decimals in the calculation of research and money.
So how does the introduction of decimals changes the calculations of all the city parameters?

If i have for example a city mainteinance of 3,80 will it be considered a mainteinance of 3 or 4?

Neither, it will be considered a maintenance of 3.80. The only rounding to the nearest gold now occurs with the grand total for your empire's maintenace (which I think still rounds down like all civ calculations). That's the whole point of the introduction of decimals, so rounding errors were less significant.

I've never seen a negative maintenance cost for a city though, and I can think of no circumstances where this should occur.

I assure you that in my capital the mainteinance is negative.I have a screenshot done with fraps but the file is too big to upload in the forum.Can anyone suggest me how to upload the file?

Isn't city maintenance always shown as a negative number, to emphasize the fact that it's money you're losing, not gaining?

I assure you that in my capital the mainteinance is negative.I have a screenshot done with fraps but the file is too big to upload in the forum.Can anyone suggest me how to upload the file?

Just make screenshots the old fashioned way, with the Print Screen button on the keyboard?

I was wrong every mainteinance is shown in negative number.