Reversal

The reversal of a decimal number is . Ball and Coxeter (1987) consider numbers whose reversals areintegral multiples of themselves. Palindromic Numbers and numbers ending with a Zero aretrivial examples.

The first few nontrivial examples are 8712, 9801, 87912, 98901, 879912, 989901, 8799912, 9899901, 87128712,87999912, 98019801, 98999901, ... (Sloane's A031877). The pattern continues for large numbers, with numbers of the form equal to 4 times their reversals and numbers of the form equal to 9 times their reversals. In addition, runs of numbers of either of theseforms can be concatenated to yield numbers of the form , equal to 4 times their reversals, and , equal to9 times their reversals.

The product of a 2-digit number and its reversal is never a Square Number except when the digits are the same(Ogilvy 1988). Numbers whose product is the reversal of the products of their reversals include

References

Ball, W. W. R. and Coxeter, H. S. M. Mathematical Recreations and Essays, 13th ed. New York: Dover, pp. 14-15, 1987.

Ogilvy, C. S. and Anderson, J. T. Excursions in Number Theory. New York: Dover, pp. 88-89, 1988.

Sloane, N. J. A. Sequence A031877in ``The On-Line Version of the Encyclopedia of Integer Sequences.''http://www.research.att.com/~njas/sequences/eisonline.html.

• Circuminscribed
• Circumradius
• Circumscribed
• Circumsphere
• Cis
• Cissoid
• Cissoid of Diocles
• Cissoid of Diocles Caustic
• Cissoid of Diocles Inverse Curve
• Cissoid of Diocles Pedal Curve
• Clairaut's Differential Equation
• Clarity
• Clark's Triangle
• Class
• Class (Group)
• Classical Groups
• Classification
• Classification Theorem
• Class Interval
• Class (Map)
• Class (Multiply Perfect Number)
• Class Number
• Class Number Formula
• Class Representative
• Class (Set)