The rules state simply that: "Mathematical formulas are allowed."

Does that mean that you can draw the word "pressure" by writing pV=nRT and underlining the letter p in the formula? I have a feeling you would get warned after the first letter so how do you make sure people understand it's a mathematical formula you're about to write?

The example in the rules is that "X + Y = Z" can be used for the word "equation".

I've always taken it to be that you can use a generic formula for words like equation, algebra, smart (http://isketch.net/instructions/gfx/a_smart.gif), maths etc but not ones like you're asking about...

Hey Jukka and welcome to the forum :)

As it directly states in the rules (http://www.isketch.net/instructions/rules.shtml), "Mathematical formulas are allowed."
I would assume that you may use any mathematical equation or scientific formula just so long as each element in the equation is substituted by one letter only.

A few examples that you may write: (especially common in the Science room)

c² = a² + b² (or the equivalent image that NH has linked)
PV = nRT
E = mc²
y = mx + c

A couple of examples you may not write:

D = Volume/Area (these are using exact words)
sin²x + cos²x = 1 (sin and cos could be abbreviations for the word)

I find it quite easy to avoid these. You could always use symbols to make sure you don't incur a vote by someone.
For example, E = mc² would become ▲ = •■². Anyone that knows the layout of these common equations should be able to realise what you are saying :)

Disclaimer: Everything about the rules I've directly mentioned in my own post above is my assumption only and should not be regarded as official.

I was once blocked from the game for writing the ideal gas law equations for the words "Ideal Gas Laws" in the Science room. Nobody warned me, I was just straight kicked. Not sure if that was for the equation or letters in general.
I think it's a bit of a grey area because the rules aren't exactly explicit. (Are mathematical formulae and scientific formulae the same thing?). I don't think you should be allowed to draw pressure in that way though. Normally you would have to press the hint button and lose potential points to reveal the first letter of the word and by writing the equation you are by-passing that.

I think it's a bit of a grey area because the rules aren't exactly explicit. (Are mathematical formulae and scientific formulae the same thing?).

Yes, it definitely is a grey area. Perhaps PV = nRT and E = mc² are regarded as scientific formulae and not mathematical formulae like y = mx + c. Maybe it's because most terms in scientific formulae use the first letters of the term they are depicting, i.e. E = energy, V = volume, P = pressure etc.

As with all grey areas regarding letters, such as on-screen personal messages or "lol" written on screen, it's best to avoid using letters at all costs; use the symbols and smileys instead :)

Okay thanks. I was just curious because I interpreted that part of the rules as if it was allowed but it would have been weird because of the general policy of not allowing letters.