How to prove following statement :

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> **Conjecture:**
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> Odd prime $p$ is expressible as : $p=x^2+4\cdot y^2$ , $x,y > 0$
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> if and only if : $p\equiv 1 \pmod {12}$ or $p\equiv 5 \pmod {12}$ .
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Similar statements (without a proof) related to the [Fermat's theorem on sums of two squares](http://en.wikipedia.org/wiki/Fermat%27s_theorem_on_sums_of_two_squares) can be found [here](http://en.wikipedia.org/wiki/Fermat%27s_theorem_on_sums_of_two_squares#Related_results) .