How to Prove All Dominoes Fall in an Infinite Arrangement
Induction Hypothesis:
Assume that the statement holds true for any arrangement of n dominoes.
Inductive Step:
Consider an arrangement of n + 1 dominoes. Since the first three fall, and the nth domino falls when the (n + 3)rd domino falls, all n + 1 dominoes fall.
Conclusion:
By strong induction, it is proven that all dominoes fall in an infinite arrangement of dominoes if you know that the first three dominoes fall, and that when a domino falls, the domino three farther down in the arrangement also falls.