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Katre, S.A. and Rajwade, A.R. 1983. On the Jacobsthal sum $\phi_9(a)$ and the related sum $\psi_9(a)$. MATHEMATICA SCANDINAVICA. 53, (Dec. 1983), 193–202. DOI:https://doi.org/10.7146/math.scand.a-12026.