A number traditionally associated with bad luck. A so-called baker's dozen is equal to 13. Fear of the number
13 is called triskaidekaphobia. There are 13
Archimedean solids. Mazur and Tate (1973/74)
proved that there is no elliptic curve over the
rationals 
having a rational
point of order 13.
13
See also
13th Root, Baker's Dozen, TriskaidekaphobiaExplore with Wolfram|Alpha
References
Mazur, B. and Tate, J. "Points of Order 13 on Elliptic Curves." Invent. Math. 22, 41-49, 1973/74.Wells, D. The Penguin Dictionary of Curious and Interesting Numbers. Middlesex, England: Penguin Books, 1986.Cite this as:
Weisstein, Eric W. "13." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/13.html