Time for another round of "when is it OK to omit the multiplication symbol?"
I think for powers of numbers, it's fine to omit, e.g.
2²3² = 36.
What about subscripts, for the number base, e.g.
12₅23₅ = 331₅,
or do I need a multiplication symbol:
12₅ × 23₅ = 331₅.
In reply to @christianp
Or, less esoterically, when I've got a subscripted variable followed by a number, e.g.
x₂4
(I will not accept "put the 4 first" as an answer)