ut3_reciprocal_weights
======================

.. py:function:: t3toolbox.backend.ut3_operations.ut3_reciprocal_weights(weights)

   .. code-block:: python

      def ut3_reciprocal_weights(
              weights: UT3WeightsData,  # (tucker_weight_supercore, tt_weight_supercore, masks)
      ) -> UT3WeightsData:              # 1/w on the real slots; padding a canonical, finite 0; masks unchanged


   Elementwise ``1/w`` on the real slots (masks unchanged) -- e.g. to form inverse-singular-value
   weights.

   **Not just ``1/w``**, because a canonical weight's padding is zero and ``1/0 = inf``, which then
   poisons every masked reduction downstream (``0 * inf = nan``). That is the headline path, not a corner
   case: the Grasedyck-Kramer metric *is* ``from_ut3svd(x).reciprocal()``. The real slots are left alone
   (see :py:func:`_ut3_map_real_weights` for how the padding is handled).

   **The real slots are deliberately unprotected**: a genuinely zero singular value gives ``inf`` here,
   exactly as in the ragged layer. It is a real weight, not a padding artifact, so it is the caller's to
   avoid -- silently clamping it would hide a rank-deficient point.