diff --git a/arithmetic_progressions/fenwick.c b/arithmetic_progressions/fenwick.c
new file mode 100644
index 0000000..d5954d1
--- /dev/null
+++ b/arithmetic_progressions/fenwick.c
@@ -0,0 +1,79 @@
+/*
+ * Author: Titouan Rigoudy
+ */
+
+#include <stdlib.h>
+
+long *fenwick_make(size_t n) {
+    long *fenwick = calloc(n, sizeof *fenwick);
+    return fenwick;
+}
+
+long *fenwick_prodmake(size_t n) {
+    long *fenwick = malloc(n * sizeof *fenwick);
+    if (fenwick == NULL) {
+        return NULL;
+    }
+    size_t i;
+    for (i = 0; i < n; i++) {
+        fenwick[i] = 1;
+    }
+    return fenwick;
+}
+
+// Add val to element i in fenwick tree
+void fenwick_update(long *fenwick, size_t n, size_t i, long val) {
+    i++;
+    if (i < 1 || i > n) {
+        return;
+    }
+    while (i <= n) {
+        fenwick[i-1] += val;
+        i += i & -i; // parent
+    }
+}
+
+// Sum of elements in [0, i[
+long fenwick_prefixsum(long *fenwick, size_t n, size_t i) {
+    long sum = 0;
+    while (i > 0) {
+        sum += fenwick[i-1];
+        i -= i & -i; // predecessor
+    }
+    return sum;
+}
+
+// Sum of elements in [i, j[
+long fenwick_intervalsum(long *fenwick, size_t n, size_t i, size_t j) {
+    return fenwick_prefixsum(fenwick, n, j) - fenwick_prefixsum(fenwick, n, i);
+}
+
+// Multiply element i by val in fenwick tree of products
+void fenwick_produpdate(long *fenwick, size_t n, size_t i, long val, long mod) {
+    i++;
+    if (i < 1 || i > n) {
+        return;
+    }
+    while (i <= n) {
+        fenwick[i-1] = (fenwick[i-1] * val) % mod;
+        i += i & -i; // parent
+    }
+}
+
+// Product of elements in [0, i[
+long fenwick_prefixprod(long *fenwick, size_t n, size_t i, long mod) {
+    long prod = 1;
+    while (i > 0) {
+        prod = (prod * fenwick[i-1]) % mod;
+        i -= i & -i; // predecessor
+    }
+    return prod;
+}
+
+// Product of elements in [i, j[
+long fenwick_intervalprod(long *fenwick, size_t n, size_t i, size_t j, long mod) {
+    long p_j = fenwick_prefixprod(fenwick, n, j, mod);
+    long p_i = fenwick_prefixprod(fenwick, n, i, mod);
+    return (p_j * INVERSES[p_i]) % BIG_MOD;
+}
+
diff --git a/arithmetic_progressions/fenwick.h b/arithmetic_progressions/fenwick.h
new file mode 100644
index 0000000..6fc42ee
--- /dev/null
+++ b/arithmetic_progressions/fenwick.h
@@ -0,0 +1,28 @@
+/*
+ * Simple Fenwick tree implementation
+ * Additive and multiplicative modulo variants
+ * Author: Titouan Rigoudy
+ */
+
+long *fenwick_make(size_t n);
+
+long *fenwick_prodmake(size_t n);
+
+// Add val to element i in fenwick tree
+void fenwick_update(long *fenwick, size_t n, size_t i, long val);
+
+// Sum of elements in [0, i[
+long fenwick_prefixsum(long *fenwick, size_t n, size_t i);
+
+// Sum of elements in [i, j[
+long fenwick_intervalsum(long *fenwick, size_t n, size_t i, size_t j);
+
+// Multiply element i by val in fenwick tree of products
+void fenwick_produpdate(long *fenwick, size_t n, size_t i, long val, long mod);
+
+// Product of elements in [0, i[
+long fenwick_prefixprod(long *fenwick, size_t n, size_t i, long mod);
+
+// Product of elements in [i, j[
+long fenwick_intervalprod(long *fen, size_t n, size_t i, size_t j, long mod);
+