Tuesday, 22 September 2026

Exercise (8.3).7

Determine three different solutions of

$$ x^2− 47y^2 = 1 $$


We first try to find the least positive solution by trying the smallest values of $y$.

Trying $y=1,2,3,4,5,6$ does not give a natural number solution for $x$. The first solution is $y=7, x=48$.


From the least positive solution $r=47+7\sqrt{47}$, we can generate two further solutions $r^2$ and $r^3$.

$$ r^2 =  4607 + 672 \sqrt{47}$$

$$ r^3 = 442224 + 64505 \sqrt{47} $$

So three solutions are $(x,y)=(48,7)$, $(x,y)=(4607,672)$ and $(x,y)=(442224, 64505 )$.