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The second perfect number. Leonhard Euler (17071783) proved that all even perfect numbers are of the form 2^{n1}(2^{n}  1) where 2^{n}  1 is a Mersenne prime M_{n}. (28#)^{2} + 29 are consecutive primes. [Luhn] 28 is a perfect number expressible as the sum of first five prime numbers, i.e., 2 + 3 + 5 + 7 + 11 = 28. [Gupta] The number of Hadamard matrices of order 28 is prime. [Rupinski] The 28th Fibonacci number plus and minus 28 is prime, i.e., F(28)28 and F(28)+28 are primes. [Opao] (28!+1)/(28+1) is a prime with 28+1 digits. [Silva] The 28th Fibonacci number plus 28^{28} is prime. This is the largest such number less than a thousand. [Opao] 28!+28^28+1 is prime. [Silva] The current score to beat in King Kong's Prime Numbers Game. Note that 28 is a perfect number. F_{n}^{2}28 is never prime, where F_{n} denotes the nth Fibonacci number. [Poo Sung] 28 is a perfect number expressible as the sum of first five nonprime numbers, i.e., 1 + 4 + 6 + 8 + 9 = 28. [Chuck]
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