Alright, the title is meant to be catchy; there is no real connection there. Except that it is in the context of paying my apartment gas bill I found out about a simple application of modulo arithmetic in electronic transactions in the US. (I am not talking about cryptography. Nowhere near that.) While trying to pay the gas bill through an e-cheque, I found that I had to fill in something called a “routing number” before submitting the e-cheque. Having never heard of the term before, I searched a bit and found out that the number serves chiefly as an identifier of a bank (or other financial institutions) in electronic transactions. It is a 9 digit number which identifies the location of a bank and the bank. (I think a bank can have multiple routing numbers.)
The modulo arithmetic part comes in in generating and validating a routing number. To validate a routing number the following checksum is used:
[3(d1 + d4 + d7) + 7(d2 + d5 + d8) + 1(d3 + d6 + d9)] mod 10 = 0
I don’t know why this particular checksum. And I think, the the first 8 digits are known and the last digit is generated using the above formula. Not sure who does this.
Incidentally, Gauss was responsible for modulo arithmetic (or clock arithmetic). The book The Music of the Primes has a nice description of this and many other beautiful things that Gauss developed. On this note, I also recall a quote in the brilliant book called Concrete Mathematics. In that book, the authors have included what are called “marginal notes” — they are actually the notes/comments written by the first set of students who got to read the draft of the book. The quote is – “It seems a lot of stuff is attributed to Gauss; either he was really smart or he had a great press agent.” On the same page, there is another marginal comment – “Maybe he just had a magnetic personality.”