RE: [math-fun] big numbers and how they're made

Quoting "Schroeppel, Richard" <rschroe@sandia.gov>:

Also c. 1965, I was coding some business program for an IBM customer, and was given a check-digit algorithm that alternately added and subtracted digits. Perhaps we took the result mod 10 to get the check; I can't recall.

Isn't this plain old ISBN? The signs alternate to get a modulo 11 sum which is immune to digit switching, a common transcription error, whereas taking a sum modulo 9 is not. - hvm ------------------------------------------------- www.correo.unam.mx UNAMonos Comunicándonos