diff --git a/collatz/GNUmakefile b/collatz/GNUmakefile
new file mode 100644
index 0000000..42d4c61
--- /dev/null
+++ b/collatz/GNUmakefile
@@ -0,0 +1,6 @@
+CFLAGS = -Wall -O9
+all: collatz collatz2
+
+collatz:
+
+collatz2:
diff --git a/collatz/collatz.c b/collatz/collatz.c
new file mode 100644
index 0000000..b1213ea
--- /dev/null
+++ b/collatz/collatz.c
@@ -0,0 +1,40 @@
+#include <stdio.h>
+#include <stdlib.h>
+
+typedef int T;
+#define FT "%d"
+
+int main(void) {
+    T i = 1;
+    long c_max = 0;
+
+    for(;;) {
+	T j = i;
+	long c = 0;
+	while (j != 1) {
+	    if (j % 2 == 1) {
+		T j2 = j * 3 + 1;
+		if ((j2 - 1) / 3 != j) {
+		    printf(FT " is not 3 * " FT " + 1 at starting point " FT, j2, j, i);
+		    exit(1);
+		} 
+		j = j2;
+	    } else {
+		j = j / 2;
+	    }
+	    c++;
+	}
+	if (c > c_max) {
+	    printf("new longest sequence starting at " FT ": %ld steps\n", i, c);
+	    c_max = c;
+	}
+
+	if (i + 1 > i) {
+	    i = i + 1;
+	} else {
+	    printf(FT " is not greater than " FT "\n", i + 1, i);
+	    exit(1);
+	}
+
+    }
+}
diff --git a/collatz/collatz2.c b/collatz/collatz2.c
new file mode 100644
index 0000000..0491c61
--- /dev/null
+++ b/collatz/collatz2.c
@@ -0,0 +1,40 @@
+#include <stdio.h>
+#include <stdlib.h>
+
+#define USE_INT 1
+
+#ifdef USE_INT
+    typedef int T;
+    #define FT "%d"
+    #define even(x) ((x) % 2 == 0)
+#elif USE_LONG
+    typedef long T;
+    #define FT "%ld"
+    #define even(x) ((x) % 2 == 0)
+#elif USE_DOUBLE
+    typedef double T;
+    #define FT "%20g"
+    #define even(x) (floor((x) / 2) == ((x) / 2))
+#endif
+
+long collatz(T j) {
+
+    long c = 0;
+    while (j != 1) {
+	printf("%ld: " FT "\n", c, j);
+	if (even(j)) {
+	    j = j / 2;
+	} else {
+	    j = j * 3 + 1;
+	}
+	c++;
+    }
+    return c;
+}
+
+int main(void) {
+    T i = 113383;
+    long c = collatz(i);
+    printf("%ld\n", c);
+    return 0;
+}