# Is there formula for this squared geometric (?) progression?

Is there non-recursive formula for the following sequence:
$$a_1=\frac12,$$ $$a_n=\frac12a_{n-1}^2+\frac12$$

If there is, how do you suggest I can determine it?

• What makes you think there is a non-recursive formula for that sequence? – Gerry Myerson Jul 23 '13 at 9:25
• $a_n-a_{n-1}=(1/2)(a_{n-1}-1)^2$ but I'm not sure that helps. – Gerry Myerson Jul 23 '13 at 9:27
• I've consulted with a mathematician who told me "ah let me think about it, it will not be too complicated" but then he did not have time to do it (in 5 minutes). And without the square it has a closed form. But I'll change my question. – Barney Szabolcs Jul 23 '13 at 9:42
• Never believe a mathematician. – Gerry Myerson Jul 23 '13 at 10:47

## 1 Answer

Making the change of variables $$a_n=1-2x_n$$ the recursion relation transforms into $$x_{n+1}=x_n(1-x_n).$$ This is a particular case of logistic map (with $r=1$). If it were solvable, I think it would be mentioned here along with $r=2,4$.

• That is very interesting! :O – Barney Szabolcs Jul 23 '13 at 10:09