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Calculus and Beyond Homework Help
Prove infinitude of primes of form 4k+1 using properties of Legendre symbol (-1/p)
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[QUOTE="Vespero, post: 3592910, member: 285051"] [h2]Homework Statement [/h2] Show that there are infinitely many primes [itex]4k+1[/itex] using the properties of [itex]\left(\frac{-1}{p}\right)[/itex]. [h2]Homework Equations[/h2] [itex]\left(\frac{-1}{p}\right) = \begin{cases} 1, & \text{if }p\equiv 1\ (mod\ 4), \\ -1, & \text{if }p\equiv 3\ (mod\ 4). \end{cases} [/itex] [itex]\left(\frac{ab}{p}\right) = \left(\frac{a}{p}\right)\left(\frac{b}{p}\right)[/itex][h2]The Attempt at a Solution[/h2] Similar to other proofs involving the infinitude of the primes, I assumed that there were only a finite number of primes of the form 4k+1, specifically [itex]p_1, p_2, ..., p_k[/itex]. I then formed the number [itex](p_{1}p_{2}...p_{k})^{2} + 1[/itex]. This number is not divisible by any [itex]p_{i}, 1 \leq i \leq k[/itex], for otherwise we would have [itex]p_{i}|1[/itex]. Now, this number has a prime factorization by the Fundamental Theorem of Arithmetic as [itex](p_{1}p_{2}...p_{k})^{2} + 1 = q_{1}q_{2}...q_{r}[/itex]. From this point on, I'm not sure where to go. Clearly, I need to incorporate the Legendre symbol, but I can't see where to apply its properties to arrive at a contradiction. [/QUOTE]
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Prove infinitude of primes of form 4k+1 using properties of Legendre symbol (-1/p)
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