interesting graph theory question from Warren Smith

[Max A.]

I do not understand why it is zero for n=2.
Consider a digraph on two nodes x,y with directed arcs (x,y) and
(y,x). To make it acyclic one needs to remove at least one arc.

Max

On 8/29/06, N. J. A. Sloane <njas at research.att.com> wrote:
>
> %I A120366
> %S A120366 0,0,0,1,1,3,5,7
> %N A120366 Max_{Directed graphs G with n nodes} minimal number of arcs that must be removed to make G acyclic.
> %C A120366 The next entry is either 8,9 or 10 (I suspect 10) and the following one is either 12,13 or 14.
> %F A120366 Always strictly increasing (except for the initial part), and (n-5)^2 / 6 <= a(n) <= (n-1)^2 / 4 .
> %O A120366 0,6
> %K A120366 nonn,more
> %A A120366 Warren D. Smith (warren.wds(AT)gmail.com), Aug 29 2006
>
> NJAS
>