On an identity for the volume integral of the square of a vector field.   Remark on a paper by A. M. Stewart
  Stewart has presented a position-space derivation of the identity for the volume integral of a product of two vector fields noted by Gubarev, Stodolsky, and Zakharov, and applied the results to classical electromagnetic theory. I present a simplified derivation suitable for use in undergraduate courses. 
Loyal Durand