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The second counting number, equal to 1+1, or, more formally, what you get when you combine the multiplicative identity with itself under addition. It is the first prime number, and the only even prime number. It is the only number apart from one to be equal to its own factorial. Operations like addition, multiplication, etc. are most naturally performed on two numbers. 2 ×2 = 2+2 = 4 is the only case of multiplication of integers have the same effect as addition. 2 is the first deficient number. The equation xⁿ+yⁿ=zⁿ has integer solutions only for n=2. So does the equation n^x+n^y=n^z. It is the third Fibonacci number. φ(2)=1,     d(2)=2,     σ(2)=3 .