Date: Sun, 8 Sep 20:01:00
From: Alex Bogomolny
Dear Dean,
n! is the product of all integers from 1 through n. For example, 2!=1*2, 3!=1*2*3, 4!=1*2*3*4, and so on. The double factorial n!! is not the same as the repeat factorial, say, (3!)!=6!=1*2*3*4*5*6. n!! is best computed backwards. It's the product of all positive integers starting with n and picking up only every other integer. For example, 6!!=6*4*2=48. 5!!=5*3*1. Therefore, I correctly claim that 31=4!!*4-4/4 since 4!!=4*2=8. Hope this helps.
Sincerely,
Alex
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