The difference between each top number is 8 (every other multiple of 4) starting with 4 either way of 0 (not inclusive). This means that any other number that is not within this category is impossible to appear at the top if a whole number is placed in the bottom left hand corner.
To find the top number straight from the bottom left hand corner number you use the formula '8n+12' or '4(2n+3)' therefore to find out the left hand corner number from the top number, you take 12 from it and then divide that number by 8.
In the first row, the differences is 1 and as the rows increase, the differences doubles e.g. in the second row the difference is 2, third row the difference is 4 and the fourth row from the borrom, the difference is 8. This is because every time you add the numbers, the differences gets added on to the previous difference within the number.
If the corner left hand number is negative then everything just ends up being a negative. The actual numbers don't change.
To find the other corner (right hand corner) you simply add 3 (unless you have more than 4 numbers at the bottom. It's basically add on to the left hand corner number how many numbers are at the bottom minus one.