'Then are equivalent' (followed by a list) in mathematical writing
As a mathematician, I find your expression looks odd, as if translated literally from some other language. The word 'Then' is normally used to introduce some statement that follows from preceding reasoning, as in 'if 1 and 1 make 2, then...'. A sentence beginning with 'Then ' is weird.
If you mean 'from the preceding material we conclude that the following statements are equivalent' then say so. If, on the other hand yo are stating a theorem, you don't need 'Then'; just say 'The following are equivalent:'