Unless you have a specific value for k the best you can do is

where (2k - 1)!! is the "odd factorial function." It multiplies only the odd numbers up to (2k - 1).

(In case you were wondering there is an even factorial function that is a product of only even numbers, and it has the same symbol, "!!" The nice thing about this is you don't really need to use it:

)

-Dan