And if I want to make in this way? Why is not possible?
In this form Mathcad attempts to run through all the n's for x before before tackling y; so when n gets to 1, y1 doesn't yet exist. The only way I've found to make it proceed recursively, is to put it in an "if" statement. (With full Mathcad Prime you could put the whole thing in a "for" loop, of course. However, I only have the Express version).