OP solved it, closing

Have you tried it on paper for a couple of inputs? The pattern is obvious after a few iterations.

+1

It means "how many pairs of integers (a,b) satisfies the system of equations".

It's perfectly clear to people who have at least been taught a bit of linear algebra.

Yes, but probably not in the way you would've initially thought.

@aoben3 how did you fix the java heap space? What was the problem? I have a similar issue.

This picture makes really good explanation.
http://glaringnotebook.com/zimages/cubism38.jpg

The big cube on the picture above has 3 cubes in a row.
Hope it helps.

A cube has six sides that are visible from outside. Your task is to count all the cubes that are inside.

Example for 3: 3x3x3 (=27) smaller cubes.

3 cubes width, 3 cubes high and 3 cubes deep.

Or 3 layers of 3x3.

First layer:
███
███
███

Second layer:
███
█X█ <--- the X isn't visible from outside
███

Third layer:
███
███
███

Hi-I had a hard time solving this too, I didn't really understand the question and had a hard time visualizing it. One thing that helped me was to look at diagrams of 2 cubed, 3 cubed, etc. You'll see that 2 cubed has zero rows in between, but that 3 cubed has 1 row between. 4 cubed has 2 rows in between. Hope this helps you a bit!

If a is [1, -1] you get infinite loop.

I had the same problem. I commented my System.out.print() (debug purpose) and heap space issue was fixed.