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▼ It's March 14, and that means only one thing … it's Pi Day and time to celebrate the world's most famous irrational number, pi. The ratio of a circle's circumference to its diameter, pi is not just irrational, meaning it can't be written as a simple fraction; it is also transcendental, meaning it's not the root, or solution, to any polynomial equation, such as x+2X^2+3 = 0.
But no so fast … pi may be one of the most well-known numbers, but for people who are paid to think about numbers all day long, the circle constant can be a bit of a bore. In fact, countless numbers are potentially even cooler than pi. We asked several mathematicians what their favorite post-pi numbers are; here are some of their answers.
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Tau
You know what's cooler than ONE pie? … TWO pies. In other words, two times pi, or the number "tau," which is roughly 6.28.
"Using tau makes every formula clearer and more logical than using pi," said John Baez, a mathematician at the University of California, Riverside. "Our focus on pi rather than 2pi is a historical accident."
Tau is what shows up in the most important formulas, he said.
While pi relates a circle's circumference to its diameter, tau relates a circle's circumference to its radius — and many mathematicians argue that this relationship is much more important. Tau also makes seemingly unrelated equations nicely symmetrical, such as the one for a circle's area and an equation describing kinetic and elastic energy.
But tau will not be forgotten on pi day! As per tradition, the Massachusetts Institute of Technology will send out decisions at 6:28 p.m. today. A few months from now, on June 28, tau will have its own day.
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Natural log base
The base of natural logarithms — written as "e" for its namesake, the 18th-century Swiss mathematician Leonhard Euler — may not be as famous as pi, but it also has its own holiday. Yup, while 3.14 is celebrated on March 14, natural log base, the irrational number beginning with 2.718, is lionized on Feb. 7.
The base of natural logarithms is most often used in equationsinvolving logarithms, exponential growth and complex numbers.
"[It] has the wonderful definition as being the one number for which the exponential function y = e^x has a slope equal to its value at every point," Keith Devlin, the director of the Stanford University Mathematics Outreach Project in the Graduate School of Education, told Live Science. In other words, if the value of a function is, say 7.5 at a certain point, then its slope, or derivative, at that point is also 7.5. And, "like pi, it comes up all the time in mathematics, physics and engineering."
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Imaginary number i
Take the "p" out of "pi," and what do you get? That's right, the number i. No, that's not really how it works, but i is a pretty cool number. It's the square root of -1, which means it's a rule breaker, as you're not supposed to take the square root of a negative number.
"Yet, if we break that rule, we get to invent the imaginary numbers, and so the complex numbers, which are both beautiful and useful," Eugenia Cheng, a mathematician at the School of the Art Institute of Chicago, told Live Science in an email. (Complex numbers can be expressed as the sum of both real and imaginary parts.)
i is an exceptionally weird number, because -1 has two square roots: i and -i, Cheng said. "But we can't tell which one is which!" Mathematicians have to just pick one square root and call it i and the other -i.
"It's weird and wonderful," Cheng said.
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i to the power of i (▪ ▪ ▪)
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