Rank-two elliptic curve with cubic x cubed minus seventeen x ($E:y^2=x^3-17x,r=2$)

The two-torsion square-class homomorphism on $E$ has candidate classes $\{1,-1,17,-17\}$ by the prime-support bound in two-isogeny descent. The identity, $(0,0)$ and $(-1,4)$ realize $1,-17,-1$, so multiplicativity gives all four. The two-isogeny formula companion $E^{\prime }:y^2=x^3+68x$ has only positive candidate classes $\{1,2,17,34\}$; $(0,0)$ realizes $[68]=17$ and $(2,12)$ realizes $2$. Thus both images have order four, and the two-isogeny rank formula gives $2^r=4\cdot 4/4=4$. The rank of an elliptic curve is therefore two, without any unresolved covering class.