Unsigned Lah numbers have an interesting meaning in combinatorics: they count the number of ways a set of elements can be partitioned into nonempty linearly ordered subsets.[3] Lah numbers are related to Stirling numbers.[4]
For , the Lah number is equal to the factorial in the interpretation above, the only partition of into 1 set can have its set ordered in 6 ways: is equal to 6, because there are six partitions of into two ordered parts: is always 1 because the only way to partition into non-empty subsets results in subsets of size 1, that can only be permuted in one way.
In the more recent literature,[5][6]Karamata–Knuth style notation has taken over. Lah numbers are now often written as
^Lah, Ivo (1954). "A new kind of numbers and its application in the actuarial mathematics". Boletim do Instituto dos Actuários Portugueses. 9: 7–15.