[stargroup100]

Originally Posted by rushyrulz

Finished problem 23 and it was an efficiency nightmare (i shud lrn 2 code b4tr)

Why do they pick 28,123 when the same is true for all numbers greater than 20,161?

I'm assuming this means that through conceptual analysis and proof, the lowest limit we can reduce down to is 28123. Even though the true limit is lower, this is the smallest upper bound we can actually prove in this way. If this is really true, I'm sure it's very well defined what they mean by "reduced any further by analysis". Clearly, brute force testing with a computer doesn't count.

Unless you're asking why the problem writer chose to use that number specifically, in which case I don't know and it's really the writer's free choice. Perhaps he just simply wanted an excuse to use larger numbers, despite the small increase. It does kinda make sense though; once you find the limit of your hand analysis, you have no choice but to use a computer, and only then can you reduce that limit. The problem probably wants to demonstrate an example of this.

I just tried this on my own and the 28123 limit is actually pretty easy to prove, and I really do have no idea how to get this down any lower.