Language: C

#include <iostream>
#include <vector>
#include <cstdio>
#include <queue>
#include <algorithm>
#include <functional>
#include <string>
#include <queue>
#include <cmath>
#include <iomanip>
#include <map>
#include <set>
#include <deque>
#include <stack>
#include <random>
#include <unordered_map>
#include <unordered_set>
#include <numeric>
#define ll long long
#define fr(i, k, N) for (decltype(N + k) i = k; i < N; i++)
#define rep(i, N) for (decltype(N) i = 0; i < N; i++)
#define ALL(v) v.begin(), v.end()
using namespace std;
template <class X>
void pr(X test) { cout << test << endl; }
template <class X>
void prr(X test)
{
    for (auto it : test)
    {
        cout << it << " ";
    }
    cout << endl;
}
template <class X>
void prrr(X test)
{
    int f = 0;
    for (auto it : test)
    {
        cout << (f++ != 0 ? " " : "") << it;
    }
    cout << endl;
}
template <typename X>
istream &operator>>(istream &i, vector<X> &v)
{
    for (X &x : v)
        i >> x;
    return i;
}

class UF
{ // union find from https://github.com/kartikkukreja/blog-codes (MIT)
    // [usage]
    // auto uf = new UF(n);
    // uf->merge(l, r); // 0 <= l, r <= n-1
    // ...
public:
    int *id; // inner id
    int cnt; // number of all vartices
    int *sz; // sz[i] means size of i's set
    UF(int N)
    {
        cnt = N;
        id = new int[N];
        sz = new int[N];
        for (int i = 0; i < N; i++)
        {
            id[i] = i;
            sz[i] = 1;
        }
    }
    ~UF()
    {
        delete[] id;
        delete[] sz;
    }
    int find(int p)
    {
        int root = p;
        while (root != id[root])
            root = id[root];
        while (p != root)
        {
            int newp = id[p];
            id[p] = root;
            p = newp;
        }
        return root;
    }
    void merge(int x, int y)
    {
        int i = find(x);
        int j = find(y);
        if (i == j)
            return;

        if (sz[i] < sz[j])
        {
            id[i] = j;
            sz[j] += sz[i];
        }
        else
        {
            id[j] = i;
            sz[i] += sz[j];
        }
        cnt--;
    }
    bool connected(int x, int y)
    {
        return find(x) == find(y);
    }
    int count()
    {
        return cnt;
    }
};

// return 約数unordered_set 1 <= x <= n
auto divv = [](auto n) {
    unordered_set<decltype(n)> v;
    for (decltype(n) i = 1; i * i <= n; i++)
    {
        if (n % i == 0)
        {
            v.insert(i);
            if (i * i != n)
                v.insert(n / i);
        }
    }
    return v;
};

// check prime (and n>=2) or not
auto isPrime = [](auto n) {
    if (n <= 1)
        return false;
    for (decltype(n) i = 2; i <= sqrt(n); ++i)
    {
        if (n % i == 0)
        {
            return false;
        }
    }
    return true;
};

// 最小公倍数
auto lcm = [](auto a, auto b) {
    if (a < b)
        swap(a, b);
    auto r = a % b;
    auto x = a * b;
    while (r != 0)
    {
        a = b;
        b = r;
        r = a % b;
    }
    return x / b;
};

unordered_map<ll, unordered_map<ll, ll>> u;
// 最大公約数
auto gcd = [](auto a, auto b) {
    if (a > b)
        swap(a, b);
    if (u[a][b] == 0)
    {
        u[a][b] = (a * b) / lcm(a, b);
    }

    return u[a][b];
};

// factorial n mod prm
ll factmod(ll a, ll prm)
{
    return a == 0 ? 1 : (a * factmod(a - 1, prm)) % prm;
};

// 1/x mod prm
auto invmod = [](auto x, auto prm) {
    decltype(x) r = 1;
    if (x == 1)
        return r;
    for (auto ex = prm - 2; ex; ex >>= 1)
    {
        if (ex & 1)
            r = (r * x) % prm;
        x = (x * x) % prm;
    }
    return r;
};

int main()
{
    ll k;
    ll sum = 0;
    cin >> k;
    fr(a, 1, k + 1)
        fr(b, 1, k + 1)
            fr(c, 1, k + 1)
                sum += gcd(c, gcd(a, b));
    pr(sum);
}


Compiler output:
main.c:1:10: fatal error: iostream: No such file or directory
    1 | #include <iostream>
      |          ^~~~~~~~~~
compilation terminated.