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# Proof Problem

Find all the integral solutions of $${ x }^{ 4 }+{ y }^{ 4 }+{ z }^{ 4 }-{ w }^{ 4 }=1995$$.

Note by Swapnil Das
1 year, 7 months ago

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We know that $$a^4 \equiv 0,1 \pmod {16} \quad \forall \ a \in \mathbb Z$$.

We can have:

$x^4 + y^4 + z^4 - w^4 \equiv -1,0,1,2,3 \pmod {16}$

or

$x^4 + y^4 + z^4 - w^4 \equiv 0,1,2,3,15 \pmod {16}$

But $$1995 \equiv 11 \pmod {16}$$. So no integral solutions exist!

@Swapnil Das - I'd say it's a Number Theory Problem! · 1 year, 7 months ago

Solution master: @Satyajit Mohanty · 1 year, 7 months ago

Yeah! It is! Great solution @Satyajit Mohanty :) · 1 year, 7 months ago

why did you work with mod16? why not other numbers? is there any rule? · 1 year, 7 months ago