Fill in subsequent rows by taking the absolute value of the number above minus the one above and to the right:
|a-b| |b-c| |c-d| |d-a| (d has nothing to the right, so we 'wrap around' and use a)

The challenge is: Find numbers a, b, c, and d that yield 20 rows where at least one number on each row is not zero. (not counting the initial row)

Example:

If we start with 1, 2, 3, 4, we get:
1 1 1 3
0 0 2 2
0 2 0 2
2 2 2 2
0 0 0 0

Originally posted by SwissGambit Start with four numbers:
a b c d

Fill in subsequent rows by taking the absolute value of the number above minus the one above and to the right:
|a-b| |b-c| |c-d| |d-a| (d has nothing to the right, so we 'wrap around' and use a)

The challenge is: [b]Find numbers a, b, c, and d that yield 20 rows where at least one number on each row is not ...[text shortened]... 2, 3, 4, we get:
1 1 1 3
0 0 2 2
0 2 0 2
2 2 2 2
0 0 0 0

Originally posted by Gastel The mistake is a b c and d are letters not numbers!!!

HA HA HA!

Who says they are letters?
He says from the beginning: "Start with four numbers: a b c d"
If he says the symbols are numbers they are, believe me. They perhaps look like letters but they are in fact numbers.

Can you solve the equation 2x=4 not realizing that the x stands for a number?