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Clay Mathematics Institute (CMI)
private nonprofit foundation dedicated to increasing and disseminating mathematical knowledge.
http://www.claymath.org/
London Mathematical Society
learned society for mathematics with a nationwide membership.
http://www.lms.ac.uk/
Math-Net
providing results of mathematical research and development, teaching material, and information about working mathematicians and mathematical institutions.
http://www.math-net.de/
Mu Alpha Theta
the National High School and Junior College Mathematics Honor Society for student mathematicians and teachers across the United States.
http://www.mualphatheta.org/
Pi Mu Epsilon
national honorary mathematics society whose purpose is the promotion of scholarly activity in mathematics among students in academic institutions.
http://www.pme-math.org/
VSAE - Universiteit van Amsterdam
student union for students in actuarial science, econometrics and operations research & management.
http://www.vsae.nl/