# Thread: theory of sequences and series

1. ## theory of sequences and series

I learnt that one precondition for y=f(x) was, that to each x-value only one single y value is assigned, so that no double solution are accepted.
Imagine now
-y(y+8)=(x-3)^2 -8. This fuction will result in a circle, which means that to some x values, two y values are assigned.
How ist that possible, respectively, did i missunderstood the rules concerning functions?

2. Originally Posted by Schdero
I learnt that one precondition for y=f(x) was, that to each x-value only one single y value is assigned, so that no double solution are accepted.
Imagine now
-y(y+8)=(x-3)^2 -8. This fuction will result in a circle, which means that to some x values, two y values are assigned.
How ist that possible, respectively, did i missunderstood the rules concerning functions?
-y(y+8)=(x-3)^2 -8 is an equation but not a function. Your condition for function is valid. I don't know what this has to do with theory of sequences and series though.