 Sparse norm preservation from entrywise Gram control

If $E=A^{T}A/d-I$ has entrywise maximum norm at most $t$, every vector $z$ supported on at most $s$ coordinates satisfies $|z^{T}Ez|\le t\Vert z{\Vert }_1^2\le st\Vert z{\Vert }_2^2$. Thus the squared norm distortion is at most $st$ on all such sparse vectors. Use the quadratic inequality at $z=0$, where a ratio would be undefined.