Symbols for "odd" and "even"

Let $A$ be a sequence of letters $\langle a,b,c,d,e,f \rangle$. I want to create two subsequences, one with the values with odd index and other with the values with even index: $A_\mathrm{odd} = \langle a,c,e \rangle$ and $A_\mathrm{even} = \langle b,d,f \rangle$.

My question is: is there any usual symbol that could substitute the words "odd" and "even" in the name of the subsequence?

Thanks!

Solution 1:

"odd" and "even" are fine. Maybe in roman not italic, though: $$ A_{\rm{odd}} \ne A_{\rm{even}} $$ since the first subscript is not a product $odd$ of three factors.

Solution 2:

The best I can come up with is $A_{2k+1}$ and $A_{2k}$.