At: http://www.research.att.com/~njas/sequences/Sindx_Coi.html under Collatz, the link is to Sindx_1, not Sindx_3 as expected. Jon Perry perry at globalnet.co.uk http://www.users.globalnet.co.uk/~perry/maths BrainBench MVP for HTML and JavaScript http://www.brainbench.com