The number of ways in which a group of 
with weights 
can change a losing coalition (one with 
) to a winning one, or vice
versa. It was proposed by the lawyer J. F. Banzhaf in 1965.
Banzhaf Power Index
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References
Paulos, J. A. A Mathematician Reads the Newspaper. New York: BasicBooks, pp. 9-10, 1995.Referenced on Wolfram|Alpha
Banzhaf Power IndexCite this as:
Weisstein, Eric W. "Banzhaf Power Index." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/BanzhafPowerIndex.html