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<h2>B-phase gradient energy and spin wave velocities</h2>

<p>See also the source file:
<a href="https://github.com/slazav/he3lib/blob/master/he3_grad.f">
<tt>he3_grad.f</tt></a>.

$$
F_\nabla = \frac12 \Delta^2 \left[
  K_1 (\nabla_j R_{ak})(\nabla_j R_{ak})
+ K_2 (\nabla_j R_{ak})(\nabla_k R_{aj})
+ K_3 (\nabla_j R_{aj})(\nabla_k R_{ak}) \right]
$$

<h3>Examples</h3>
<p>
<script>
  plot