I'm creating the concept for a story called The Little Differences. It's about an accountant that, one day, out of the blue, notices that a certain calculation is producing a slightly wrong result. Barely noticeable, nothing world-changing,

He runs it on the computer, tries different software, a physical calculator... everything gives a result that's a little off. When he checks on paper himself, he gets the correct result. But, to his surprise, everyone else tells him that he's the one that's off, and that the incorrect result is actually perfectly sound.

I need something that makes sense, mathematically. The weird result must be something that really is wrong, and not just something that programs sometimes get wrong (I don't want it to be explained at all... I mean, the reason why it is occurring must not be something easily reducible to some well-known malfunction). But it must also be minor enough for someone to miss, something that wouldn't really cause much trouble in the real world (is that possible? IDK).

Lastly: it must be something that I'm able to explain (on some level) to a non-math reader.

So, Tildes math wizzes, what you suggest? :D

I know those are easy to find on children's education materials, but the keyword here is simple. Anything kiddie, requiring too many clicks or that takes 3 seconds is off the table.

I kinda need to brush up on my super basic reasoning. In part, because a grown man should know the time table by heart, and in part, because I wanna convince myself that I did not get brain damage after being hit by a car (there's really nothing indicating that, I'm just paranoid).

I'm looking for something simple that presents me with the multiplication table and or simple calculations to answer under a time frame. The requirement for iOS is that my computer is broken.

And it really can be super simple, even something I open from a mobile web browser or transfer to my Kindle.

Additionally: there is no need for if to be an app. A huge list of exercises (with answers) would be greatly helpful.

Thanks!

I'm interested in the subject, but don't know where to begin investigating it. I tried to look over the code for SeDuMi, but it is much more massive than I had realized. I have a background in mathematics, if anyone can point me towards a textbook.