Casting Out Nines, Nine-Point Center, Nine-Point Circle, Nine-Point Conic, Rule of Nines, Wigner 9j-Symbol
Wells, D. The Penguin Dictionary of Curious and Interesting Numbers. Middlesex, England: Penguin Books, pp. 73-76, 1986.
Eric W. Weisstein. "9."
From MathWorld--A Wolfram Web Resource. http://mathworld.wolfram.com/9.html
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