Langford Sequence Origins

Skolem sequences and the closely related Langford sequences were discovered independently by mathematicians Thoralf Skolem and Dudley Langford in 1957 and 1958. The story goes that Langford was inspired by watching his kid play with colored blocks, observing that there was one block between the red blocks, two blocks between the blue blocks and three blocks between the green blocks:

Langford realized the sequence of blocks if replaced with numbers would read: 312132.