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I have a problem with

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\startitemize[columns,a] \item $a_n=2+\frac{1}{n+1}$ \item $a_n=\frac{(-1)^n}{\sqrt{n}}-3$ \stopitemize

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Hi,

this did not wotk because the second formula is than the first and -------------------------------------------------^ (what?)

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the multicolumn OTR tries to put half or less of the text into the second column, you can now force a second column with the \column command after the first line or you switch both formulas and the OTR can make two columns.

\starttext

\startitemize[columns,a] \item $a_n=\frac{(-1)^n}{\sqrt{n}}-3$ \item $a_n=2+\frac{1}{n+1}$ \stopitemize

Why is this working? Here the first column is as big as the second above (same formula), so the same problem should arise with the first column as above with the second?

No, they are not, the first row (column) is 1 points taller than the second one, you can test this with TeX.

\starttext

\setbox\scratchbox\hbox{$a_n=\frac{(-1)^n}{\sqrt{n}}-3$}

Formula 1: \copy\scratchbox

Height of formula 1: \the\ht\scratchbox

\setbox\scratchbox\hbox{$a_n=2+\frac{1}{n+1}$}

Formula 2: \copy\scratchbox

Height of formula 2: \the\ht\scratchbox

Ahhh! The HEIGHT (not the width) ist the problem!? So: \startcolumns \dontleavehmode \vphantom{$a_n=\frac{(-1)^n}{\sqrt{n}}-3$}$a_n=2+\frac{1}{n+1}$% $a_n=\frac{(-1)^n}{\sqrt{n}}-3$ \stopcolumns works too. Is there something like an \mathstrut macro? (I know, that there exists a macro with this name, but that uses ')' as height.)

I hope my examples helped you a little bit to understand the problems in your example and why my methods worked.

Wolfgang

Hope, I have it! Thank you, Wolfgang