# computability – Are all \$Sigma^0_1\$ sets of infinite sequences infinite?

I think I figured out some things about $$Sigma^0_1$$ and $$Pi^0_1$$ in the arithmetical hierarchy, for sets of infinite sequences, and I’m hoping I can get confirmation that I’m right, or can understand the ways in which my thinking is incorrect.

What I think I have figured out is that every set of infinite sequences in $$Sigma^0_1$$ must be infinite in size–in fact uncountable–since a sentence containing only existential quantifiers over indexes into a sequence can only require that some digits satisfy a predicate. Therefore, any sequence satisfying the sentence would do because of a property of a finite number of locations in the sequence. All subsequent digits after those locations would be allowed to vary freely, in which case the number of sequences with the same initial pattern would be uncountable.

By contrast, sets of infinite sequences in $$Pi^0_1$$ may be either be finite or infinite, I think. For example:

$${x: (forall n), x(n)=0}$$ contains only one element, $$000ldots$$ .

$${x: (forall n), x(n)=0$$ if $$n$$ is odd$$}$$ allows the digits at the even-numbered locations to vary freely, so the number of sequences that satisfy this predicate for all $$n$$ is uncountable.

Is this correct? Are there ways in which I’m confused, or some obvious nuance that I’m missing?