diff --git a/13-functions.c b/13-functions.c
new file mode 100644
index 0000000..0204ff1
--- /dev/null
+++ b/13-functions.c
@@ -0,0 +1,24 @@
+#include <stdio.h>
+
+int power(int m, int n);
+
+/* test power function */
+int main()
+{
+	int i;
+
+	for (i = 0; i < 10; ++i)
+		printf("%d %d %d\n", i, power(2,i), power(-3,i));
+	return 0;
+}
+
+/* power: raise base to n-th power; n >= 0 */
+int power(int base, int n)
+{
+	int i, p;
+
+	p = 1;
+	for (i = 1; i <= n; ++i)
+		p = p * base;
+	return p;
+}