Is it Euler’s identity eiπ + 1 = 0

where e is Euler's number (2.718), the base of natural logarithms, i is the imaginary unit, which satisfies i2 = -1, and π (3.14) is the ratio of the circumference of a circle to its diameter. Probably the numbers 0, 1, e, π and i are the most used numbers in mathematics. These 5 numbers play important and recurring roles across mathematics, and are the five constants appearing in the above Euler's identity.

Or could it be one of such seminal theorems as

(a) The number of primes is infinite;

(b) There is no rational number whose square is 2;

(c) π is transcendental;

(d) A continuous mapping of the closed unit disk into itself has a fixed point.

David Wells, the writer of a number of popular mathematical works, suggested for a mathematical statement to be beautiful it must be simple, brief, important, and, obvious when it is stated but perhaps easy to overlook otherwise, surprising. Lots of mathematician think Euler’s theorem (eiπ + 1 = 0) scores high on all four counts. What is your favorite theorem? What others you may think as beautiful as Euler's theorem?

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