# Putting normal letters in between Greek ones

So I am trying to write out Gauss' law for gravitation and I am having a hard time figuring out how to write it out without an error.

I am currently trying to write in an equation block

4\piG\rho\delta


but obviously TeX is picking up on the fact that I have \piG and not \pi. I can delineate between the two by adding a \  to space them out but that looks funky in the equation. Any help is appreciated.

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What makes G any more "normal" than π? – TRiG Mar 28 '14 at 11:04

Spaces in math mode are ignored and replaced with the appropriate surrounding space required for each component (like a relational or binary operator, or atom). So, you should be fine with

4 \pi G \rho \delta


although technically 4\pi G\rho\delta would suffice.

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As an aside, π should be written in upright style, using e.g. \piup from txfonts package, instead of \pi. (By standards, symbols of mathematical constants are written in upright style, not italic, and this is generally applied in physics, though very often violated in mathematics.) – Jukka K. Korpela Mar 28 '14 at 6:50
@JukkaK.Korpela Depends on your subject area: pure mathematicians don't use upright for pi/e/d, while more applied areas do at least for d. (Yes, I know that the ISO says they should all be upright.) – Joseph Wright Mar 28 '14 at 6:52
From my physics reading I would say the rule is obeyed for "e", "d" and other Latin alphabet "constants", but the glyph given by \pi is the most common symbol for 3.14... for work produced in LaTeX and even some quite old books. – Chris H Mar 28 '14 at 7:53
I've never heard of that before and therefore used \pi and so on, where can this ISO be found? wikipedia only covers subscripts, symbols for units and numbers (en.wikipedia.org/wiki/ISO_31-0#Mathematical_signs_and_symbols). – Verena Haunschmid Mar 28 '14 at 8:40
It’s ISO 80000-2 (and IEC 80000-2) now, available from usual ISO document distributors. And as I wrote, though mathematicians have their own habits, in physics the standard is generally followed, and the formula here is clearly a physics formula. – Jukka K. Korpela Mar 28 '14 at 9:02