diff --git a/number/get_phi.cc b/number/get_phi.cc
new file mode 100644
index 0000000..96a4d85
--- /dev/null
+++ b/number/get_phi.cc
@@ -0,0 +1,28 @@
+vector<int> get_phi(int n) {
+    vector<bool> not_prime(n + 1);
+    vector<int> res;
+    vector<int> phi(n + 1);
+    phi[1] = 1;
+    for (int i = 2; i <= n; ++i) {
+        if (not not_prime[i]) {
+            res.emplace_back(i);
+            phi[i] = i - 1;
+        }
+        for (auto&& x : res) {
+            if (i * x > n) break;
+            not_prime[i * x] = 1;
+            if (i % x == 0) {
+                // phi(n) = n * prod((p - 1) / p)
+                // => phi(i * x) = i * x * prod((p - 1) / p) = (i * prod((p - 1) / p)) * x = phi(i) * x,
+                // since `i` covers all factors of i * x
+                phi[i * x] = phi[i] * x;
+                break;
+            } else {
+                // i coprimes x
+                // phi(i * x) = phi(i) * phi(x) = phi(i) * (x - 1)
+                phi[i * x] = phi[i] * (x - 1);
+            }
+        }
+    }
+    return phi;
+}