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For an integer $n$, the semi-factorial $n!!$ can be defined as $$ n!! = n(n-2)(n-4)\cdots $$ In other words, the semi-factorial of $n$ is the familiar factorial, but with every other term omitted. For example, $6!! = 6 \cdot 4 \cdot 2$.

This might be generalized slightly as follows: For an integer $n$, the "$k$-semi-factorial" of $n$ is the familiar factorial, but with every $k^\text{th}$ term omitted. For example, the "$3$-semi-factorial" of $6$ would be $6 \cdot 5 \cdot 3 \cdot 2$.

Is the concept of "$k$-semi-factorial" already named? If so, how is it generally denoted?

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    I would take 6!! to be $6\cdot 4 \cdot 2$, not $5 \cdot 3 \cdot 1$ [Mathworld](http://mathworld.wolfram.com/DoubleFactorial.html) agrees, but [Wikipedia](http://en.wikipedia.org/wiki/Factorial#Double_factorial) thinks it questionable, though it cites this as one possibility.2012-10-12
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    @RossMillikan I think you are correct about the traditional semi-factorial. I've updated my question.2012-10-12

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