User blog comment:King2218/Extended Factorial Notation/@comment-10429372-20140209182927/@comment-25418284-20140210003955

Wythagoras, your definition is incorrect. It's $$1!! = 1$$ and $$(n + 1)!! = \text{expfac}(n) \cdot n$$ (exponential factorial).

King, I like your function, but your choice of notation is a little confusing because it coincides with the multifactorial function. Maybe change it to $$n^!_!$$ or something like-'at.