Hello, This is a Langford number : 231213 Between the two 1's there is 1 digit Between the two 2's there are 2 digits Between the two 3's there are 3 digits. This is an almost-Langford number : 62312136 Between the two 1's there is 1 digit Between the two 2's there are 2 digits Between the two 3's there are 3 digits Between the two 6's there are 6 digits The difference between a Langford and an almost- Langford number is that the first one shows all pairs of integers from 1 to n (here 1 to 3) -- which is not the case for the second one. What could be the biggest almost-Langford number? This number cannot have more than 20 digits as pairs of digits must be unique -- your theoretical alphabet to write this number is: 00112233445566778899 Best, É. (hope this is not old-hat)