code: def d2AIbr_dV2(dIbr_dVa, dIbr_dVm, Ibr, Ybr, V, lam):
      
      diaglam = spdiag(lam)
      diagIbr_conj = spdiag(conj(Ibr))
  
      Iaa, Iav, Iva, Ivv = d2Ibr_dV2(Ybr, V, diagIbr_conj * lam)
  
      Haa = 2 * ( Iaa + dIbr_dVa.T * diaglam * conj(dIbr_dVa) ).real()
      Hva = 2 * ( Iva + dIbr_dVm.T * diaglam * conj(dIbr_dVa) ).real()
      Hav = 2 * ( Iav + dIbr_dVa.T * diaglam * conj(dIbr_dVm) ).real()
      Hvv = 2 * ( Ivv + dIbr_dVm.T * diaglam * conj(dIbr_dVm) ).real()
  
      return Haa, Hav, Hva, Hvv
docs: Computes 2nd derivatives of |complex current|**2 w.r.t. V.