From: "David Wilson"
Subject: Manual method for computing A008904(n)
Date: Wed, 19 May 2010 17:58:07 -0400
A method for computing the last nonzero digit of n! = A008904(n)
If n = 0 or 1, then A008904(n) = 1.
For n >= 2, do as in this example with n = 1492.
1) Convert n to base 5
21432
2) Multiply each digit of (1) by its place, and sum the result
2*4 + 1*3 + 4*2 + 3*1 + 2*0 = 22
3) Compute half the sum of just the even digits of (1)
(2 + 4 + 2)/2 = 4
4) Sum (2) and (3)
22 + 4 = 26
5) Compute remainder mod 4
26 mod 4 = 2
6) Compute power of 2
2^2 = 4
7) If result is 1, change it to 6.
4
So the last nonzero digit of 1492! is 4.