7→22→11→34→17→52→26→13→40→20→10→5→16→8→4→2→1→4→2→1→4→2→1...

Edit: also, 9→28→14→7...

And, 15→46→23→70→35→106→53→160→80→40→20→10→5→16→8→4→2→1...

Or even, 19→58→29→88→44→22→11→...

And lest we forget, 3→10→5→16→8→4→2→1...

That covers every odd number below 20. Want me to do 21 and 25, too? Perhaps 27?