[1] O’Beirne, T.H. (1961) Puzzles and Paradoxes. New Scientist, No. 230, 53-54
[2] Gardner, M. (1987) The Second Scientific American Book of Puzzles and Diversions. University of Chicago Press, Chicago.
[3] Trott, M. (2004) The Mahematica Guide Book for Programming. Springer-Verlag, New York.
http://dx.doi.org/10.1007/978-1-4419-8503-3
[4] Ghannam, T. (2012) The Mystery of Numbers: Revealed through Their Digital Roots. 2nd Edition, Create Space Publications, Seattle.
[5] Dudley, U. (1978) Elementary Number Theory. Dover, New York.
[6] Pritchard, C. (2003) The Changing Shape of Geometry: Celebrating a Century of Geometry and Geometry Teaching. Cambridge University Press, Cambridge.
[7] Hinden, H.J. (1974) The Additive Persistence of a Number. Journal of Recreational Mathematics, 7, 134-135.
[8] Averbach, B. and Orin, C. (2000) Problem Solving through Recreational Mathematics. Dover Publications, Mineola.
[9] Polya, G. (1957) How to Solve It: A New Aspect of Mathematical Method. 2nd Edition, Princeton University Press, Princeton.
[10] Noller, R.B., Ruth, E.H. and David, A.B. (1978) Creative Problem Solving in Mathematics . State University College at Buffalo, Buffalo.