除法分块求区间约数和

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LL get_sum(LL n){
    LL ans = 0;
    for(LL l = 1,r;l <= n;l = r + 1){
        r = n / (n / l);
        ans += (n / l) * (l + r) * (r - l + 1) / 2;
        }
    return ans;
    }