# On This Day in Math – March 13

Many Paris Street signs are named for Mathematicians *The n-Category CafeIt is my task to convince you not to turn away because you don’t understand it. You see my physics students don’t understand it… That is because I don’t understand it. Nobody does.~Richard FeynmanThe 72nd day of the year; 72 is a pronic, heteromecic, or oblong number (and sometimes pronic is spelled promic). They are numbers that are the product of two consecutive integersOblong numbers have the property that if they are used in infinite nested radicals, they produce an integer, (sqrt(72+sqrt(72+sqrt(72+…))) = 9 )Newton categorized  62 cubic curves in 1710.72 is the smallest number whose fifth power is the sum of five smaller fifth powers: (19^5 + 43^5 + 46^5 + 47^5 + 67^5 = 72^5).The rule of 72 was once a commonly used approximation in banking and finance for the time it took an investment to double at r%. For a 5% investment, the approximate period would be 72/5 = 14.4 years. The rule applies to compound interest. The rule is based on an approximation of ln(2) = .693.. In typography, point sizes are measured in 1/72 of an inch, 72-point characters are 1 inch tall. 72 is the smallest number… 