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short communications
In the paper by Sánchez-Bajo & Cumbrera [J. Appl. Cryst. (1997), 30, 427–430], there was an error in equation (11): the letter k appeared in the place of η. The correct equation (11) is as follows: W(σ) = β−1[−2ηw3 arctan(σ/w) + 2ηw2σ + 2( 1 − η)F(σ)]. (11)