Consider the Laplace operator $\Delta = d^{*} d + d d^{*}$ on $\Omega^2(S^3)$. What is the minimal eigenvalue of $\Delta$?
(My computations showed that the answer is 4; the eigenforms correspond to forms with constant coefficients in $R^4$. Later I found papers that claim that answer is 3, but I don’t understand what the eigenforms would be in that case.)