Prove that when n=1, xn=1, the proposition holds. Now assume n>1, and let xn=x. Clearly, x=1, and we have xn=x−1xn−1, which means
xn+1−2xn+1=0
From (1), we get xn(2−x)=1, so x1-x n -x (1-x n ) = (1-x n )(1-x)>0
This is a contradiction, so x⩾1.
Next, x⩾2−n+12. In fact, if xn1(2−x+nx−1) (this is because when the sum of two numbers is a constant 2−x+nx, the smaller the smaller number, the smaller the product), i.e.,
x(2−x)>2−x+nx−1
Repeating this process, we get
xn(2−x)>xn−1(2−x+nx−1)>⋯>2−x+nx−1⋅n=1
This contradicts (1), which proves that x⩾2−n+12.
For y>x⩾2−n+12, we have ny>nx⩾2−x, so
nx(2−x)>ny(2−x+ny−x)
Therefore,
xn(2−x)>xn−1y(2−x+ny−x)>⋯>yn(2−x+ny−x⋅n)=yn(2−y)
Thus, if x(2−2n−11)n⋅2n−11=2(1−2n1)n>1$
This contradicts (1), so x⩾2−2n1.
This proves the required inequality.