[20251003] BOJ / G2 / 시그마 함수 / 권혁준

khj20006/202510/03 BOJ G2 시그마 함수.md

@@ -0,0 +1,34 @@
+```cpp
+#include <bits/stdc++.h>
+using namespace std;
+using ll = long long;
+
+ll N;
+
+unordered_set<ll> m;
+bitset<1000001> e;
+vector<ll> p = {2};
+
+void bck(ll n) {
+    for(ll i:p) {
+        if((__int128)n*i > (__int128)N) break;
+        if(m.count(n*i)) continue;
+        m.insert(n*i);
+        bck(n*i);
+    }
+}
+
+int main(){
+    cin.tie(0)->sync_with_stdio(0);
+
+    for(int i=2;i*i<=1000000;i++) if(!e[i]) for(int j=i*i;j<=1000000;j+=i) e[j] = 1;
+    for(ll i=3;i<=1000000;i++) if(!e[i]) p.push_back(i*i);
+
+    cin>>N;
+    m.insert(1);
+    bck(1);
+
+    cout<<N-m.size()<<'\n';
+
+}
+```