Hello Math-Fun, Here is another stupid iteration - which leads nowhere, probably. Start with (my date of birth) for instance: 1951 This in "p"; now make the absolute differences between p's digits, from left to right, like this: 1-9 = 8 9-5 = 4 5-1 = 4 1-1 = 0 (this is the last digit of "p" minus the first one) You then get "q" = 8440. And now: if q > p add q to p and iterate If q < p subtract q from p and iterate. Here: 1951 + 8440 ------- 10391 step 1 + 13680 ------- 24071 step 2 + 24761 ------- 48832 step 3 - 40512 ------- 8320 step 4 - 5128 ------ 3192 step 5 - 2871 ------ 321 step 6 - 112 ----- 209 step 7 + 297 ----- 506 step 8 + 561 ----- 1067 step 9 + 1616 ------ 2683 step 10 + 4251 ------ 6934 step 11 - 3612 ------ 3322 step 12 - 101 ------ 3221 step 13 - 1012 ------ 2209 step 14 - 297 ------ 1912 step 15 + 8811 ------- 10723 step 16 ... Etc. Don't know where this goes: loops? Fixed points? Patterns? Errors above? Waste of time? (yes) Best, É. [Color enhanced: http://www.cetteadressecomportecinquantesignes.com/AddOrSubtract.htm]