Let Si be a sequence in a set A.

Prove that Si has a constant subsequence iff there exists a in A such that for all n is in the natural numbers, there exists j≥n such that Si=a

Where I am confusing myself is with the j and the n. Is the n referring to the position of the subsequence in the original sequence?

I understand that a sequence does not need to be constant in order have a constant subsequence.

Where do i start?