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, commonly used to show the base of the representation, e.g.

12₅23₅ = 331₅,

or do I need a multiplication symbol:

12₅ × 23₅ = 331₅.

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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)

@christianp Something like this comes up when you deal with hypergeometric functions, e.g. $$x\,{}_2 F_1\left({{a,b}\atop{c}}\mid x\right)$$ .

@tpfto what's the ambiguity there?

@christianp it isn't ambiguous in that particular example, but I've seen work where someone had the bright idea to also use subscripted variables along with those, which are likely to confuse novices.

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