# CAS Registry Numbers algorithm

*From*: Jacobo Cruces Colado <qocruces { *at * } usc.es>
*Subject*: CAS Registry Numbers algorithm
*Date*: Sat, 19 May 2001 14:57:29 +0100

Dear all:

`Searching the archives of CHMINF-L { *at * } LISTSERV.INDIANA.EDU (URL
is
``http://listserv.indiana.edu/archives/chminf-l.html), I found
out the
``algorithm used to generate the checksum number of every CAS RN. Since
it
``could be of interest for some people in the list, I will reproduce the
text
``here (thanks to the original sender, Gary Wiggins; I hope not to break
any
``rule by reproducing this and giving the source):
` ALGORITHM: The last digit is a checksum digit of the unit integer of
the summed products of multiplying the left-next-most integer by 1, followed by
multiplying the left-next-most integer by 2, etc.
e.g.'s:
Acetone: RN= 67-64-1
1X4 + 2X6 + 3X7 + 4X6 = 61, and 1 is the checksum digit of RN=67-64-1.
Butane: RN= 106-97-8

` 1X7 + 2X9 + 3X6 + 4X0 + 5X1 = 48, and 8 is the checksum digit
of
``RN=106-97-8
`

`However, other people has pointed out this algorithm to be defective:
due
``to our decimal notation system, the same checksum number can be
obtained
``for a transposed number.
` Comments??