diff --git a/project_euler/Problem 401/sol1.c b/project_euler/Problem 401/sol1.c
new file mode 100644
index 00000000..b555a3c2
--- /dev/null
+++ b/project_euler/Problem 401/sol1.c	
@@ -0,0 +1,116 @@
+#include <stdio.h>
+#include <time.h>
+#include <stdlib.h>
+#include <stdint.h>
+#ifdef _OPENMP
+#include <omp.h>
+#endif
+
+#define MOD (uint64_t)1e9
+#define MAX_L 5000
+
+char is_in(uint64_t N, uint64_t *D, uint64_t L)
+{
+    uint64_t i;
+    for (i = 0; i < L; i++)
+        if (D[i] == N)
+            return 1;
+    return 0;
+}
+
+uint64_t get_divisors(uint64_t N, uint64_t *D)
+{
+    uint64_t q, r;
+    int64_t i, num = 0;
+
+    if (N == 1)
+    {
+        D[0] = 1;
+        return 1;
+    }
+
+    for (i = 1; i * i <= N + 1; i++)
+    {
+        r = N % i;
+
+        if (r == 0)
+        {
+            q = N / i;
+            if (!is_in(i, D, num))
+            {
+                D[num] = i;
+                num++;
+            }
+            if (!is_in(q, D, num))
+            {
+                D[num] = q;
+                num++;
+            }
+        }
+        if (num == MAX_L)
+            D = (uint64_t *)realloc(D, MAX_L * sizeof(uint64_t) << 1);
+    }
+    return num;
+}
+
+/**
+ * sum of squares of all integer factors
+ **/
+uint64_t sigma2(uint64_t N)
+{
+    uint64_t sum = 0, DD, L;
+    int64_t i;
+    uint64_t *D = (uint64_t *)malloc(MAX_L * sizeof(uint64_t));
+
+    L = get_divisors(N, D);
+    for (i = 1; i < L; i++)
+    {
+        DD = (D[i] * D[i]) % MOD;
+        sum += DD;
+    }
+
+    free(D);
+    return sum % MOD;
+}
+
+/**
+ * sum of squares of factors of numbers
+ * from 1 thru N
+ **/
+uint64_t sigma(uint64_t N)
+{
+    uint64_t s, sum = 0, i;
+
+#ifdef _OPENMP
+#pragma omp parallel for reduction(+ \
+                                   : sum)
+#endif
+    for (i = 0; i <= N; i++)
+    {
+        s = sigma2(i);
+        sum += s;
+    }
+    return sum % MOD;
+}
+
+int main(int argc, char **argv)
+{
+    uint64_t N = 1000;
+
+    if (argc == 2)
+        N = strtoll(argv[1], NULL, 10);
+    else if (argc > 2)
+    {
+        fprintf(stderr, "Wrong number of input arguments!\n");
+        return -1;
+    }
+
+    clock_t start_time = clock();
+    uint64_t result = sigma(N);
+    double dtime = clock() - start_time;
+
+    printf("N = %llu\nSum: %llu\n", N, result);
+    printf("Time taken: %.4gms\n", dtime * 1e3 / CLOCKS_PER_SEC);
+
+    return 0;
+}
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