From: "David Wilson" <davidwwilson(AT)comcast.net>
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.