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Ponder This

October 1999

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Ponder This Challenge:

Background and notation:
A "directed graph" consists of a collection of "vertices" {A,B,C,....} and "edges" {(B,C),...}.
An edge goes from one vertex to another: (B,C) goes from the source B to the destination C. There may or may not be another edge from C to B. In the present problem no edge will go from a vertex to itself (loops), nor will two edges have the same source and same destination (multiple edges).
A "path of length two" from B to C is a pair of edges (B,D) and (D,C) where the destination of the first edge is the source of the second.

Our particular directed graph has between 30 and 40 vertices, inclusive. For any two nodes B and C, there is either an edge (B,C) or exactly one path of length two from B to C, but not both.
This is true even if C=B: there is exactly one path of length two from B to B.

The question: How many vertices are in the graph?

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Challenge: 10/01/99 @ 12:00 AM EST
Solution: 11/01/99 @ 12:00 AM EST
List Updated: 11/01/99 @ 12:00 AM EST

People who answered correctly:

Alamaru Krishal/Krishna (10.1.1999 @ 7:48 PM EST)
Lawrence T. Hoff (10.1.1999 @ 11:17 PM EST)
Aleksandar Basaric (10.2.1999 @ 6:13 AM EST)
Alan Nash (10.2.1999 @ 5:10 PM EST)
Jimmy Hu (10.4.1999 @ 2:28 AM EST)
Brett Maune (10.4.1999 @ 10:11 PM EST)
H.P.Ravikiran (10.8.1999 @ 10:23 AM EST)
John D. Valois (10.10.1999 @ 11:25 PM EST)
Mani Aulakh (10.11.1999 @ 5:24 AM EST)
David Radcliffe (10.12.1999 @ 6:49 PM EST)
David Morley (10.12.1999 @ 11:14 PM EST)
msk (10.15.1999 @ 12:04 PM EST)
"algor" (10.17.1999 @ 1:46 PM EST)
Meg Meek (10.17.1999 @ 5:14 PM EST)
Mihai Cioc (10.18.1999 @ 9:46 AM EST)
Ibrahim Okuyucu (10.21.1999 @ 12:43 PM EST)
Gundars Kokts (10.26.1999 @ 5:19 PM EST)
Ming-Wei Wang (10.26.1999 @ 6:16 PM EST)
Dave Biggar (10.28.1999 @ 10:11 AM EST)
Surender Baswana (10.28.1999 @ 5:21 AM EST)
Michael Moyer (10.30.1999 @ 4:43 PM EST)

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