Atanassov, K. "On the 22nd, 23rd, and the 24th Smarandache Problems. Notes on Number Theory and Discrete Mathematics, Sophia, Bulgaria5, 80-82, 1999.Atanassov, K. On Some of the Smarandache's Problems. Lupton, AZ: American Research Press, pp. 16-21, 1999.Sloane, N. J. A. Sequence A007913 in "The On-Line Encyclopedia of Integer Sequences."Smarandache, F. Only Problems, Not Solutions!, 4th ed. Phoenix, AZ: Xiquan, 1993.