  The main result of this note is an efficient presentation of the
$S^1$-equivariant cohomology ring of Peterson varieties (in type $A$) as a
quotient of a polynomial ring by an ideal $\mathcal{J}$, in the spirit of the
well-known Borel presentation of the cohomology of the flag variety. Our result
simplifies previous presentations given by Harada-Tymoczko and Bayegan-Harada.
In particular, our result gives an affirmative answer to a conjecture of
Bayegan and Harada that the defining ideal $\mathcal{J}$ is generated by
quadratics.
