Eisenstein has done something extraordinary. His proof actually has nothing to do with QR, other than a method for building the right exponent to go on -1, though it does involve the primes p and q. It looks at the ratios of p and q when the prime in the numerator is multiplied by successive even numbers under the modulus of the other, with a chop function appended. In other words a function that always rounds down instead of moving to the nearest value. It would have been sufficient to find the correct parity under any circumstance, but Eisenstein is more exact than that, producing the exact exponent on -1 as the number of lattice points in the prescibed regions of his p by q rectangle. Only the (p-1) by (q-1) part interests him, containing the interior lattice points of the larger rectangle, and then only those with even coordinates.
This proof is wonderfully clever. The downside is that it will not reveal any deeper properties of numbers that help elucidate why QR works. Deeper investigations might uncover why it works. It proves what it proves--that Eisenstein's method will always find the right exponent for -1 in the Legendre symbol.


Reply With Quote
