Around Boston and Cambridge, there are a bunch of billboards with computational problems from a software company (ITA Software) with the challenge "Solve This... Work here. The problem that interested me was:
Write a program to compute the sum of all the integers between 1 and 10^11 that are both divisible by seven and, when the decimal digits are reversed, are still divisible by seven. Solutions to this problem will be judged on performance or algorithmic sophistication.
I found a neat solution (i.e. program) that solves this problem in a fraction of a second. The solution is interesting because you don't need to find all the integers in order to add them.
My solution (with java source code) is now posted on my web page. I would like to discuss the problem with others who have worked on it.