We calculate enough terms to discover the pattern,
Here are the first dozen terms:
a(1) = 2001
a(2) = 2002
a(3) = 2003
a(4) = 2001+2002-2003 = 2000
a(5) = 2002+2003-2000 = 2005
a(6) = 2003+2000-2005 = 1998
a(7) = 2000+2005-1998 = 2007
a(8) = 2005+1998-2007 = 1996
a(9) = 1998+2007-1996 = 2009
a(10) = 2007+1996-2009 = 1994
a(11) = 1996+2009-1994 = 2011
a(12) = 2009+1994-2011 = 1992
...
We want to find a(2004)
That's an even term. We look at the even terms:
n a(n)
--------
2 2002
4 2000
6 1998
8 1996
10 1994
12 1992
We see that the pattern for the even terms is
"if you add n and a(n), the sum is always 2004."
That is n + a(n) = 2004, or,
a(n) = 2004-n when n is even
So when n = 2004, a(2004) = 2004-2004 = 0
Answer: 0
Edwin