# Re: Cannot use some greek letters

On 2014-05-06, Steve Burnham wrote:

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> Forum,

> I am currently writing my thesis in LyX and am writing a number of
> equations, many of which use greek letters.  All has been working fine
> until today when I needed to use the greek letter “phi”.  When trying
> to use that letter I get “could not find LaTeX command for character
> “phi” (code point 0x3d5).  I am using the latest LyX on OSX with
> MacTeX.  In the description of the error it says "Some characters of
> your document are probably not representable in the chosen encoding.
>  Changing the document encoding to utf8 could help.”  I tried changing
> my encoding to utf8 but it just breaks the document even more and
> doesn’t like the other greek letters.  Any advice on how to get around
> this?

Please tell, how you input the letters, which engine you use and (if
Xe/LuaTeX) if you use unicode-math.

Remember, that there are two small phi symbols in Unicode: while in Greek
text, these are just variant glyphs of the same character, in Maths they are
treated as separate symbols.

The safe way should be to input the symbols via their TeX math-macro:

\phi and \varphi

(works only in a math inset). It is good style, to put all variables in a
math inset, even if used inside a sentence like:

The variables $r$ and $\varphi$ describe the position of $\vec{c}$ in polar
coordinates.

(This is how it should look in the LaTeX preview and source file.)

Günter