73

This number is a prime.

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The first difficult prime to form under the rules of the four 4's puzzle. The problem is to find expressions for numbers using exactly four 4's and a finite number of mathematical symbols and operators in common use. One version of the rules allow only addition, subtraction, multiplication, division, square root, exponentiation, factorial, decimal point, parentheses, as well as concatenation. Here are two possible solutions (achieved by altering the rules above):