"(a + b) \times (a - b) = a^2 - b^2...The difference of two square numbers is always a product, and divisible both by the sum and by the difference of the roots of those two squares; consequently the difference of two squares can never be a prime number."
— Prime number

January 1, 1970

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Integers
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Leonhard Euler, Elements of Algebra (1810) 2nd ed., Vol. 1 p. 117. [Editor's note:] This theorem is not general, for when the difference of the two numbers is 1, and their sum is a prime, it is evident that the difference of the two squares is also a prime: thus 6^2 - 5^2 = 11, \; 7^2 - 6^2 = 13, \; 9^2 - 8^2 = 17, &c. In fact every prime number is the difference of two integral squares.