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double alpha
In mathematics, the double factorial or semifactorial of a number n (denoted by n!!) is the product of all the integers from 1 up to n that have the same parity (odd or even) as n. That is,
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{\displaystyle n!!=\prod _{k=0}^{\left\lceil {\frac {n}{2}}\right\rceil -1}(n-2k)=n(n-2)(n-4)\cdots }
(A consequence of this definition is that 0!! = 1, as an empty product.)
Therefore, for even n the double factorial is
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{\displaystyle n!!=\prod _{k=1}^{\frac {n}{2}}(2k)=n(n-2)(n-4)\cdots 4\cdot 2\,,}
and for odd n it is
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{\displaystyle n!!=\prod _{k=1}^{\frac {n+1}{2}}(2k-1)=n(n-2)(n-4)\cdots 3\cdot 1\,.}
For example, 9!! = 9 × 7 × 5 × 3 × 1 = 945.
The double factorial should not be confused with the factorial function iterated twice, which is written as (n!)! and not n!!.
The sequence of double factorials for even n = 0, 2, 4, 6, 8,... starts as
1, 2, 8, 48, 384, 3840, 46080, 645120,... (sequence A000165 in the OEIS)The sequence of double factorials for odd n = 1, 3, 5, 7, 9,... starts as
1, 3, 15, 105, 945, 10395, 135135,... (sequence A001147 in the OEIS)Meserve (1948) (possibly the earliest publication to use double factorial notation) states that the double factorial was originally introduced in order to simplify the expression of certain trigonometric integrals arising in the derivation of the Wallis product. Double factorials also arise in expressing the volume of a hypersphere, and they have many applications in enumerative combinatorics. They occur in "Student's" t-distribution (1908), though Gosset did not use the double exclamation point notation.
The term odd factorial is sometimes used for the double factorial of an odd number.
Switching over to a bullet feeder, so no longer can use this.
Used for about 2500 rounds, and runs flawless.
$55 Or best offer.
Will trade for 600 Small Pistol or Small Rifle Primers (must be newer; if they are older primers, 800 primers...I know this all comes down to storage, but have been...
***SPF***
Looking to sell as a bundle. 34" belt, x4 mag pouches, DAA PDR Pro holster for Glock 34.
Specs on labels, I'm unfamiliar with this gear.
Asking $150 for all. Local FTF, prefer meeting near Canby.
I have a like new Primer Pro Primer Tube Filler for small primers both pistol and rifle, includes 5 magnetic collars for primer tubes, works great, I really like this machine but was gifted a Dillon and really can’t give that back (forgot to take it off the wish list) I like this machine better...
Complete Rig to shoot USPSA. Holster, mag pouches, inner and outer belt are all Double Alpha. Holster was bought to fit a Glock G17 Gen5. Belt size is 42. Located in Oregon City.
This is a Double Alpha Race Master Holster for the STI grips, it is in good shape with lots of wear left in it, so if you are looking for a nice holster for your race gun you dont have to look any farther. 125.00 plus shipping!