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f(1)=1/n
f(2)=1-(1-f(1))/(n-1) =(n-1)/n
f(x:x>2)=f(x-1)-(1-f(x-1))/(n-(x-2))
f(x:x>2)=(n-(x-2))/(n-(x-3))-(1-(n-(x-2))/(n-(x-3)))/(n-(x-2))
f(x:x>2)=((n-(x-2))*(n-(x-2))-1)/((n-(x-3))*(n-(x-2)))
f(x:x>2)=((n-(x-3))*(n-(x-1)))/((n-(x-3))*(n-(x-2)))
f(x:x>2)=(n-x+1)/(n-x+2)
if(n>1) f(n)=(n-n+1)/(n-n+2)=1/2
else f(1)=1/n=1