= Solution
Constant $c$ and $\lambda$ preserve the six spatial Euclidean isometries, namely three <spatial translations> and three <spatial rotations>. They also preserve the <de Sitter dilation> $(\tau,\mathbf x)\mapsto(s\tau,s\mathbf x)$, under which an equal-time correlator transforms covariantly together with its observation time. For generic $c\ne1$, the preferred propagation speed breaks the three <special conformal transformations>. The correlators therefore obey seven of the ten <de Sitter isometries>. When $c=1$, the action is fully de Sitter invariant and all ten are restored.
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