integer f(n)（数论）
This time I need you to calculate the f(n) . (3<=n<=1000000)f(n)= Gcd(3)+Gcd(4)+…+Gcd(i)+…+Gcd(n).Gcd(n)=gcd(C[n][1],C[n][2],……,C[n][n-1])C[n][k] means the number of way to choose k things from n some things.gcd(a,b) means the greatest
