TITLE: Best way to denote some trigonometric functions ("tg" vs "tan", "ctg" vs "cot")
QUESTION [9 upvotes]: What is the best way to denote tangent and other trigonometric functions: tg or tan, ctg or cot. What notation is commonly used and standardized?

REPLY [8 votes]: Standard
ISO 80000-2 Quantities and units — Part 2: Mathematical signs and symbols to be
used in the natural sciences and technology
is perfectly clear: the right symbols are $\tan x$ (item 2-13.4) and $\cot x$ (item 2-13.5).  
To cite the standard: "$\text{tg } x$, $\text{ctg } x$ should not be used."