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Two exclamation marks, skipping every other factor.
Also known as: Double factorial, Semifactorial, Odd factorial
Two exclamation marks mean multiply down in steps of two rather than one, so the odd or even numbers alone. It is not the factorial applied twice, which is the error the notation invites and the reason it is worth a record.
Denotes the product of every second integer down from the argument — the odd numbers if it is odd, the even ones if it is even.
Contexts: Mathematics
Is not the factorial of a factorial, despite the doubled mark, and the two give different values for every argument above two.
Contexts: Mathematics
Two upright tapering strokes each with a dot beneath, set side by side immediately after a numeral.